Methods and systems for interferometric analysis of surfaces and related applications

ABSTRACT

A method for determining a spatial property of an object includes obtaining a scanning low coherence interference signal from a measurement object that includes two or more interfaces. The scanning low coherence interference signal includes two or more overlapping low coherence interference signals, each of which results from a respective interface. Based on the low coherence interference signal, a spatial property of at least one of the interfaces is determined. In some cases, the determination is based on a subset of the low coherence interference signal rather than on the entirety of the signal. Alternatively, or in addition, the determination can be based on a template, which may be indicative of an instrument response of the interferometer used to obtain the low coherence interference signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 USC §120, this application is a continuation of andclaims the benefit of U.S. application Ser. No. 12/262,375, filed Oct.31, 2008, now U.S. Pat. No. 7,586,620, which is a continuation of U.S.application Ser. No. 11/942,166, filed Nov. 19, 2007, now U.S. Pat. No.7,456,975, which is a continuation of U.S. application Ser. No.10/941,649, filed Sep. 15, 2004, now U.S. Pat. No. 7,298,494, whichclaims the benefit of U.S. provisional application Nos. 60/502,932,filed Sep. 15, 2003; 60/502,933, filed Sep. 15, 2003; 60/502,907, filedSep. 15, 2003; 60/502,930, filed Sep. 15, 2003; and 60/539,437, filedJan. 26, 2004. The contents of the prior applications are incorporatedherein by reference in their entirety.

FIELD OF THE INVENTION

The invention relates to interferometric analysis of objects, such as tointerferometric determination of an object topography.

BACKGROUND

Interferometry, e.g., scanning white light interferometry (SWLI), may beused to determine a spatial property of an object. Typical spatialproperties include a surface topography or location of the object withrespect to some reference. For objects including a thick film overlyingan opaque substrate, the SWLI data may include two spaced partinterference patterns resulting, respectively, from the substrate-filminterface and film-air interface. If the interference patterns areentirely separable, i.e., if there is a region of zero modulationbetween the two signals, then the data can provide independentinformation about the substrate surface and film-air interface usingstandard techniques. As the overlying film becomes thinner, therespective interference patterns begin to overlap and distort oneanother. Such overlapped interference patterns can provide erroneousspatial information regarding the substrate surface and film-airinterface.

SUMMARY

One aspect of the invention relates to methods and systems for analyzinglow coherence interference signals from objects producing interferencepatterns that overlap as a function of optical path length difference(OPD). Methods and systems of the invention may be used in, e.g., flatpanel display measurements, semiconductor wafer metrology, solder bumpprocessing, in situ thin film measurements, and dissimilar materialsanalysis.

One exemplary method relates to the rapid determination of a spatialproperty of a photoresist film over a patterned wafer for in-situ focusand tilt adjustments with respect to a photolithography system. Thespatial property can include a topography and/or a position of a topsurface of the photoresist with respect to a reference of thephotolithography system. In some embodiments, the spatial property isindicative of an absolute or relative position of the photoresist film,e.g., with respect to the photolithography system.

In general, in one aspect, the invention features a method including:(i) obtaining a low coherence interference signal from a measurementobject, the measurement object including first and second interfaces,the low coherence interference signal including first and secondoverlapping interference patterns respectively resulting from the firstand second interfaces; and (ii) identifying a subset of the overlappinginterference patterns, the subset having a greater contribution from oneof the first and second interference patterns than the otherinterference pattern.

Embodiments of the method may include any of the following features.

The obtaining may include obtaining a plurality of low coherenceinterference signals, each low coherence interference signal includingfirst and second overlapping interference patterns, each firstinterference pattern resulting from a different point of the firstinterface, each second interference pattern resulting from a differentpoint of the second interface, the obtaining a plurality of lowcoherence interference signals including imaging the object, and theidentifying a subset includes identifying a subset of each of theinterference signals, each subset having a greater contribution from oneof the first and second interference patterns of the correspondinginterference signal than the other interference pattern.

For each of the plurality of low coherence interference signals, theobtaining may include using an interferometer, each of the plurality oflow coherence signals resulting from light have a range of optical pathlength differences, each range of optical path length differences beingat least 50% of a coherence length of the interferometer. For example,each range may be at least as great as the coherence length of theinterferometer.

The first interface may be an outer surface of the object and the methodmay include determining a relative height of each of a plurality of thepoints of the outer surface.

The first and second interfaces may be separated by 1000 nm or less.

The first interface may be an outer surface of the object and the secondinterface is beneath the outer surface. For example, the outer surfacemay be an outer surface of a layer of photoresist overlying a substrateand the second interface is defined between the outer surface of thephotoresist and the substrate.

The first and second interfaces may be interfaces of a liquid crystaldisplay cell.

The method may further include determining a spatial property of each ofat least some of the points of the first or second interfaces based onthe low coherence interference signals.

Each of the first and second overlapping interference patterns mayinclude a plurality of fringes and the determining a spatial property ofeach of at least some of the points may include determining the spatialproperty of each of at least some of the points based on less thanone-half of the fringes one of the first or second overlappinginterference patterns.

Each of the first and second overlapping interference patterns mayinclude a plurality of fringes and the determining a spatial property ofeach of at least some of the points may include transforming theone-half of the fringes of each first or second overlapping interferencepattern.

The determining a spatial property of each of at least some of thepoints may include determining the spatial property of each of the atleast some points based on an asymmetric subset of the overlappinginterference patterns of the corresponding interference signal. Forexample, the determining a spatial property of each of at least some ofthe points may include transforming the asymmetric subset. Also, thefirst interface may be an outer surface of the object and thedetermining a spatial property of each of at least some of the pointsmay include determining a spatial property of a plurality of points ofthe outer surface. In such cases, each asymmetric subset may bedominated by interference resulting from the outer surface of theobject.

The identifying a subset may include determining a boundary of theasymmetric subset based on a template indicative of an interferometerresponse. For example, the template may be indicative of anobject-height independent interferometer response. The determining aboundary may includes comparing the template and the low coherenceinterference signal.

The identifying a subset may include cross-correlating a templateindicative of an interferometer response and the low coherenceinterference signal. The cross-correlating may include normalizing basedon a shape of the low coherence interference signal.

The determining a spatial property of each of at least some of thepoints may include determining the spatial property based on thecorresponding low coherence interference signal and a templateindicative of an interferometer response. The determining a spatialproperty of each of at least some of the points may further includecomparing the corresponding low coherence interference signal and thetemplate. The comparing may include determining a location of best matchbetween the corresponding low coherence interference signal and thetemplate. For example, the comparing may include cross-correlating thetemplate and the overlapping interference patterns. Thecross-correlating may include normalizing based on a shape of thecorresponding low coherence interference signal.

The template may be asymmetric. For example, template may have a shapeof a truncated interference pattern. The method may further includegenerating the template by obtaining a reference low coherenceinterference signal from each of a plurality of points of a referenceobject, wherein the template includes contributions derived from each ofthe reference low coherence interference signals.

For example, each reference low coherence interference signal mayinclude object-height dependent properties indicative of a height of thecorresponding object point, and preparing the template may includeremoving object-height dependent properties from the reference lowcoherence interference signals. The reference low coherence interferencesignals may include a non-overlapping interference pattern.

In another aspect, the invention features an apparatus, including: (i)an optical system configured to obtain a low coherence interferencesignal from an object, the object having a plurality of interfaces,wherein the low coherence interference signal includes at least firstand second overlapping interference patterns resulting from at leastfirst and second interfaces of the object; and (ii) a processorconfigured to determine a spatial property of at least one of the firstand second interfaces based on an asymmetric subset of the first andsecond overlapping interference patterns.

Embodiments of the apparatus may include any of the following features.

The processor may further configured to determine a portion of the lowcoherence interference signal including greater contributions from oneof the first and second interference patterns as opposed to the other ofthe interference patterns, and select the subset of data from theportion of the low coherence interference signal.

The first interface may be an outer surface of the object and the firstinterference pattern results from the first interface, and the processormay be further configured to determine a spatial property of the firstinterface.

The optical system may be configured to obtain a plurality of lowcoherence interference signals from the object, each low coherenceinterference signal including respective first and second overlappinginterference patterns resulting from different points of the first andsecond interfaces, and wherein the processor may be further configuredto determine a spatial property of a plurality of points of the firstinterface based on respective asymmetric subsets of the plurality of lowcoherence interference signals.

For example, the spatial property of the plurality of points may be arelative height of each of the points.

The processor may be configured to determine a spatial property of theouter surface of the object with respect to another object.

The processor may be configured to determine a position of the outersurface of the object relative to a photolithography apparatus.

The processor may be further configured to carry out any of the stepsdescribed above with respect to the first-mentioned method aspect of theinvention.

In general, in another aspect, the invention features a processorconfigured to at least: receive a low coherence interference signalobtained from an object using an interferometer, the object having aplurality of interfaces, wherein the low coherence interference signalincludes at least first and second overlapping interference patternsresulting from at least first and second interfaces of the object; anddetermine a spatial property of at least one of the first and secondinterfaces based on an asymmetric subset of the first and secondoverlapping interference patterns.

Embodiments of the processor may further include features describedabove with respect to the first-mentioned method aspect of theinvention.

In general, in another aspect, the invention features a method fordetermining a spatial property of an object, the method including: (i)providing a low coherence interference signal obtained from a firstobject using an interferometer; and (ii) determining a spatial propertyof the first object based upon the low coherence interference signal anda template indicative of a response of the interferometer.

Embodiments of the method may include any of the following features

The first object may include an outer surface and the determining mayinclude determining a relative height of a point of the outer surface.

The providing may include providing a plurality of low coherenceinterference signals each obtained from a corresponding different pointof the first object using the interferometer; and the determining mayinclude determining a spatial property of each of the different pointsbased upon the corresponding low coherence interference signal and thetemplate.

The plurality of low coherence interference signals may be each obtainedby steps including imaging a portion of the object using theinterferometer.

The object may have an outer surface and the spatial property of each ofthe different points is a height of each point. For example, the objectmay include a wafer coated with photoresist and the outer surface of theobject may be an outer surface of the photoresist.

The template may be asymmetric. For example, the template may have ashape of a truncated interference pattern. The low coherenceinterference signal may include overlapping interference patternsresulting from first and second interfaces of the object and thedetermining a spatial property may include determining a spatialproperty of at least one of the first and second interfaces. The firstinterface may be an outer surface of the object and the determining mayinclude determining a spatial property of the outer surface.Furthermore, a shape of the template may corresponds to a portion of thelow coherence interference signal dominated by contributions from thefirst interface as opposed to the second interface.

The template may include contributions derived from each of a pluralityof second low coherence interference signals. For example, each of theplurality of second low coherence interference signals may result from arespective, different point of a reference object.

Each of the plurality of second low coherence interference signals mayinclude object-height dependent properties indicative of a height of thecorresponding reference object point, and wherein the method includesgenerating the template, and the generating the template may includeremoving contributions from the object-height dependent properties. Forexample, each second low coherence interference signal may have aphase-related property indicative of the height of the correspondingreference object point and the removing contributions may includeremoving the phase-related property. Also, the first object and thereference object may be the same. Alternatively, the first object mayinclude a substrate including an overlying thin film and the pluralityof second low coherence interference signals may be obtained from aportion of the reference object having a single reflective interface.

The method may further including comparing the template and the lowcoherence interference signal.

The comparing may include cross-correlating the template and the lowcoherence interference signal. For example, the cross-correlation mayinclude a partially complex cross-correlation. The cross-correlating mayinclude normalizing based upon a shape of the low coherence interferencesignal.

The comparing may include determining a location within the lowcoherence interference signal and the determining a spatial property mayinclude processing a portion of the low coherence interference signallocated to one side of the location. Furthermore, the method may alsoinclude transforming the low coherence interference signal. For example,the low coherence interference signal may include first and secondoverlapping interference patterns and the portion of the low coherenceinterference signal to the one side of the location may be dominated bycontributions from the first interference pattern as opposed to thesecond interference pattern. For example, the first interference patternmay result from an outer surface of the first object.

In general, in another aspect, the invention features a method forpreparing an interferometer template, including: providing a pluralityof low coherence interference signals, each low coherence interferencesignal having been obtained from a corresponding different point of anobject using an interferometer; and generating a template indicative ofa response of the interferometer, the generating including combiningcontributions from each of the low coherence interference signals.

Embodiments of the method may include any of the following features.

Each low coherence interference signal may have been obtained from thecorresponding different point of the object by imaging a portion of theobject using an interferometer.

Each of the different points of the object may have a respective spatialproperty and each low coherence interference signal may include aspatial dependent property dependent on the spatial property of thecorresponding object point and the generating the template may includeremoving the spatial dependent property of at least some of the lowcoherence interference signals.

The spatial property may be a relative height.

The removing the spatial dependent property may include: transformingthe low coherence interference signals to an inverse dimension, thetransformed low coherence interference signals exhibiting a phase changealong the inverse dimension; and removing a linear portion of the phasechange with respect to the inverse dimension.

The different points of the object may be different points about aninterface of the object.

The interface may be an outer surface of the object.

The method may further include: obtaining, with the interferometer, atleast one second low coherence interference signal, the second lowcoherence interference signal resulting from a point of a second object;and determining a spatial property of the point of the second objectbased on the second low coherence interference signal and the template.

For example, the second object may include a substrate and at least oneoverlying layer, the point of the second object defined by at least aportion of the overlying layer. The overlying layer may have a surfacethat defines an outer surface of the object, the point of the secondobject being located at the outer surface.

The obtaining, with the interferometer, may include obtaining aplurality of second low coherence interference signals, each second lowcoherence interference signal resulting from a different point of thesecond object and the determining a spatial property of the point of thesecond object may include determining a spatial property of thedifferent points of the second object based on the second low coherenceinterference signals and the template.

The obtaining a plurality of second low coherence interference signalsmay include imaging a portion of the second object.

The determining may include comparing the second low coherenceinterference signal and the template. For example, the comparing mayinclude cross-correlating the at least one second low coherenceinterference signal and the template. Furthermore, the comparing mayinclude normalizing the cross-correlation with respect to a shape of thefirst low coherence interference signals.

The template may have the form of an asymmetric truncated low coherenceinterference signal.

In general, in another aspect, the invention features a method forgenerating an interferometer template, including: providing at least onelow coherence interference signal, the low coherence interference signalincluding an interference pattern resulting from at least one point ofan object and having been obtained using an interferometer; andgenerating, from the at least one low coherence interference signal, anasymmetric template indicative of a response of the interferometer.

Embodiments of the method may include any of the following features.

The asymmetric template may have a shape of a truncated interferencepattern.

The at least one point of the object may have a spatial property and theat least one low coherence interference signal may have a spatialdependent property dependent on the spatial property of the at least onepoint, and wherein the generating may include removing the spatialdependent property from the low coherence interference signal.

In general, in another aspect, the invention features a method,including: providing a low coherence interference signal obtained froman object; and determining a spatial property of the object based on acomparison of the low coherence interference signal and a template, thetemplate having a shape of a truncated interference pattern.

Embodiments of the method may include any of the following embodiments.

The providing may include providing a plurality of low coherenceinterference signals, each obtained from a corresponding different pointof the object; and the determining a spatial property may includedetermining a spatial property of each of the different points of theobject based on a comparison of the corresponding low coherenceinterference signal and the template.

The method may further include comparing the low coherence interferencesignal and the template by cross-correlating the low coherenceinterference signal and the template.

The object may further include a substrate and at least one overlyinglayer and the spatial property of the object is a spatial property of atleast a point of an outer surface of the overlying layer.

The low coherence interference signal may have been obtained by a methodincluding reflecting light from the outer surface of the overlyinglayer. For example, the overlying layer may be a photoresist.

In general, in another aspect, the invention features an interferometer,including: an optical system configured to obtain a plurality of lowcoherence interference signals from different points of an object; and aprocessor including code. The processor configured to prepare a templateindicative of a response of the interferometer, the template includingcontributions from the plurality of low coherence interference signals.

Embodiments of the apparatus may include any of the following features.

The plurality of low coherence interference signals may includeproperties related to spatial properties of the different points of theobject; and the code configured to prepare a template, may include codeconfigured to prepare a template lacking at least some of the propertiesrelated to spatial properties of the different points of the object.

The processor including code is further configured to: obtain a secondlow coherence interference signal from a second object; and determine aspatial property of the second object based on the second low coherenceinterference signal and the template. For example, the code to determinea spatial property may include code configured to compare the second lowcoherence interference signal and the template.

The processor may be further configured to carry out any of thecorresponding steps described above in connection with theabove-described method aspects.

In general, in another aspect, the invention features a processorincluding computer readable medium, the medium including code configuredto cause the processor to: receive a plurality of low coherenceinterference signals obtained from different points of an object usingan interferometer; and generate a template indicative of a response ofthe interferometer, the template including contributions from theplurality of low coherence interference signals.

In further embodiments, the code may further cause the processor tocarry out nay of the corresponding steps described above in connectionwith the above-described method aspects.

An embodiment of system for obtaining interferometry data includes alow-coherence (spectrally broadband and/or extended source)interferometer, e.g. a scanning white light interferometry (SWLI)instrument, equipped to mechanically or electro-optically scan at leastan element of the system so as to change an optical path difference(OPD) between a reference and measurement path, the measurement pathbeing directed to an object surface. A processor records a plurality ofinterference signals during the OPD scan, such as by imaging a portionof the object onto a plurality of detector elements. By virtue of thelow coherence source and/or geometry of the apparatus, the interferencesignal is localized about the zero OPD position for each imaged point ofthe object surface. The system is configured to analyze objects that mayhave multiple layers, in particular a transparent thin film on asubstrate, for which the interference signals from the interfacesbetween layers are not entirely separated.

A method of the invention includes selecting a portion of theinterference signal of detected by each detector element. The selectedportion corresponds to a selected interface or surface, e.g., aninterface between a substrate and thin film layer. The selected portionof each interference signal is identified as being the relativelyundistorted by unwanted interference phenomena resulting fromreflections from the other interfaces or surfaces. The selected portionmay include or be limited to interference information acquired for OPDpositions different from the zero OPD for the interface of interest.

An inventive method for selecting portions of interference signalsincludes extracting a portion of the interference signal starting at aparticular optical path length difference identified e.g. by the signalcentroid or some other characteristic of the signal that predictablydefines the limits of the substantially uncorrupted portion of thesignal of interest. The extracted portion is then sent for furtherprocessing as if it were the total signal.

Another inventive method for selecting portions of low coherenceinterference signals includes comparing measurement interference signalsacquired from a measurement object and template generated by priormeasurement of a reference surface, by using the measurementinterference signals themselves, by theoretical prediction orcombination thereof. The template can correspond to a portion of eachlow coherence interference signal that is substantially undistorted. Themethod can include finding the location of each measurement lowcoherence interference signal that best matches the template. Thistechnique can provide the interface height location directly, withoutfurther processing. As an improvement to the measurement resolution, thetechnique may include more than one stored signal, for example a set oftemplates with various offsets, so as to allow for improvedinterpolation.

As a step in the data processing, once the location of the interface hasbeen identified according to the location of the selected signal portionwith respect to optical path length difference, compensating factors maybe included so as to account for refractive and dispersive properties ofthin films and how these material parameters relate e.g. to the NA ofthe objective and spectral bandwidth of the illumination, so as toprovide corrected data corresponding to the true physical profile of theinterface of interest.

In some embodiments, a spatial property of an outer surface ofphotoresist is determined based on at least a portion of an interferencesignal including overlapping interference patterns. A relative positionof the photoresist and a photolithography system can be modified basedon the spatial property.

In some embodiments, a spatial property of an outer surface of an objectis determined from at least a portion of a low coherence interference,e.g., after removing some material from the outer surface. Based on thespatial property, additional material can be removed. For example, arate of removal can be modified during the additional removal.

In some embodiments, a spatial property of a portion of an object isdetermined, e.g., after irradiating the portion of the object with alaser beam to form a scribe line. Additional scribing of the object oranother object is performed based on the spatial property.

In some embodiments, a plurality of low coherence interference signalsis obtained. Each interference signal includes detector intensity as afunction of optical path length difference values and can be designatedas I_(sys). The plurality of interference signals are averaged in thefrequency domain to determine a single, partial spectrum {tilde over(q)}_(sys) corresponding to the field average over all the low coherenceinterference signals in the frequency domain.

The partial spectrum is inverse transformed to provide a templateĨ_(sys) , the real part of which may represent interference signalssimilar to each interference signal, but with amplitude scaling andobject height dependent differences removed. In some embodiments, thetemplate is retained in a complex form Ĩ_(sys) such that the envelopeand phase at each scan position ζ can be separated using the modulus andargument, respectively, of the complex function Ĩ_(sys) .

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs.

Unless otherwise stated, spatial properties of objects determined bymethods and systems discussed herein may be relative or absolute.

Other features, objects, and advantages of the invention will beapparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a low-coherence interference signal from a solid siliconsubstrate without a thin film or other coating. The scan position is theζ coordinate.

FIG. 2 is a template including contributions from interference signalsobtained from each of a plurality of different locations of the siliconsubstrate referring to in the description of FIG. 1. The template ofFIG. 2 has been multiplied by a window function, which is also shown.

FIG. 3 illustrates overlapping interference patterns obtained from anobject having a substrate and an overlying thin film.

FIG. 4 is an asymmetric template.

FIG. 5 is a low-coherence interference signal from a (silicon) substratewith a 2-μm coating of Si₃N₄.

FIG. 6 is a low-coherence interference signal from a (silicon) substratewith a 1.1-μm coating of Si₃N₄.

FIG. 7 is an expanded view of a direct comparison of the plot in FIG. 3(diamonds) and the plot in FIG. 6 (solid line).

FIG. 8 is a flowchart illustrating a method for preparing a template.

FIG. 9 is an exemplary interferometry system for obtaining interferencesignals.

FIG. 10 a is a cross-section of a measurement object including asubstrate and an overlying layer, e.g., a thin film.

FIG. 10 b is a top view of the object of FIG. 1 a.

FIGS. 11 a and 11 b are exemplary structures having copperinterconnects. FIGS. 11 a and 11 b show the structure before and afterplanarization, respectively.

FIGS. 12 a and 12 b are exemplary structures formed during solder bumpprocessing. FIG. 12 a shows the structure before addition of solder.FIG. 12 b shows the structure after addition of solder but prior toflowing the solder.

FIG. 13 is a portion of an exemplary liquid crystal display.

FIG. 14 a illustrates a positive-frequency portion of the magnitudespectrum of the Fourier transform of the interference signal of FIG. 2.

FIG. 14 b is a template in the transformed domain including apositive-frequency portion of a field-averaged spectrum {tilde over(q)}_(sys) . The template includes contributions from a plurality ofinterference signals from different points of an object.

FIG. 15 illustrates a template determined from a plurality ofinterference signals and also illustrates the envelope under whichfringes of the template decay. The template is identical to the templateof FIG. 3 except for not having been subjected to a window function.

FIG. 16 illustrates the product of the amplitude of the fringes andenvelope of the interference signal of FIG. 2.

FIG. 17 illustrates a merit function determined from the windowedtemplate of FIG. 3 and the interference signal of FIG. 2.

FIG. 18 illustrates height profiles of the object used to acquire theinterference signal of FIG. 2.

FIG. 19 illustrates one of a second set of 101 simulated referenceinterference signals I_(ex) acquired from the silicon substrate used toacquire the signal of FIG. 2. The scan position is the ζ coordinate.

FIG. 20 a illustrates a positive-frequency portion of the magnitudespectrum of the Fourier transform of the interference signal of FIG. 19.

FIG. 20 b illustrates template transformed interferometry data and is apositive-frequency portion of a field-averaged spectrum {tilde over(q)}_(sys) including contributions from a plurality of interferencesignals according to FIG. 19.

FIG. 21 is an asymmetric template determined from the product of atemplate derived from the data of FIG. 19 and an asymmetric windowingfunction. Only the right-hand portion of the template derived from thedata of FIG. 19 has been retained.

FIG. 22 illustrates one of 101 simulated interference signals I_(ex)acquired from a silicon substrate having a thin film. Other than thepresence of the thin film, the silicon substrate is the same as thatused to acquire the data of FIG. 19. The scan position is thecoordinate.

FIG. 23 illustrates the product of the amplitudes of the fringes andenvelopes of the interference signal of FIG. 22.

FIG. 24 illustrates a merit function determined from the asymmetrictemplate of FIG. 21 and the data of FIG. 22.

FIG. 25 illustrates height profiles of the substrate-film interface ofthe silicon substrate used to acquire the data of FIG. 22 as determinedusing the template of FIG. 21 and the data of FIG. 22.

DETAILED DESCRIPTION

Referring to FIG. 1, a simulated low coherence interference signal 150includes a plurality of detector intensity values obtained from a singlepoint of an object, e.g., a point of a silicon wafer having a singlereflective interface. The intensity values are plotted as a function ofan optical path length difference (OPD) between light reflected from theobject point and light reflected from a reference object. Interferencesignal 150 is a low coherence scanning white light interferometry (SWLI)signal obtained by scanning the OPD, e.g., by moving an optic and/or theobject to vary the optical path traveled by the light reflecting fromthe object or the reference light. A Mirau interferometer is an exampleof an interferometer that can be configured as a low coherence scanningwhite light interferometer. An interferometer may, alternatively or incombination, vary the OPD by detecting a spatial distribution of lightreflected from the object and the reference light with the OPD varyingas a function of spatial position on a detector.

In FIG. 1, the intensity values are plotted as a function of OPD (herescan position) and map out an interference pattern 151 having aplurality of fringes 152, which decay on either side of a maximumaccording to a low coherence envelope 154. In the absence of a lowcoherence envelope, the fringes of an interference pattern typicallyhave similar amplitudes over a wide range of optical path differences.The envelope 154 itself does not expressly appear in such interferencesignals but is shown for discussion. The location of the interferencepattern along the OPD axis is generally related to a position of zeroOPD, e.g., a scan position or spatial position corresponding to zero OPDbetween light reflected from the object point and from a referenceobject. The zero OPD scan position is a function of the objecttopography, which describes the relative height of each object point,and the orientation and position of the object itself, which influencesthe position of each object point with respect to the interferometer.The interference signal also includes instrumental contributions relatedto, e.g., the interferometer optics, e.g., the numerical aperture (NA)of the optics, the data acquisition rate, the scan speed, thewavelengths of light used to acquire the interference signal, thedetector sensitivity as a function of wavelength, and other instrumentalproperties.

The width of the coherence envelope 154 that modulates the amplitudes offringes 152 corresponds generally to the coherence length of thedetected light. Among the factors that determine the coherence lengthare temporal coherence phenomena related to, e.g., the spectralbandwidth of the source, and spatial coherence phenomena related to,e.g., the range of angles of incidence of light illuminating the object.Typically, the coherence length decreases as: (a) the spectral bandwidthof the source increases and/or (b) the range of angles of incidenceincreases. Depending upon the configuration of an interferometer used toacquire the data, one or the other of these coherence phenomena maydominate or they may both contribute substantially to the overallcoherence length. The coherence length of an interferometer can bedetermined by obtaining an interference signal from an object having asingle reflecting surface, e.g., not a thin film structure. Thecoherence length corresponds to the full width half maximum of theenvelope modulating the observed interference pattern.

As can be seen from FIG. 1, interference signal 150 results fromdetecting light having a range of optical path differences that variesby more than the width of the coherence envelope and, therefore, by morethan the coherence length of the detected light. In general, a lowcoherence interference signal can result from obtaining interferencefringes that are amplitude modulated by the coherence envelope of thedetected light. For example, the interference pattern may be obtainedover an OPD for which the amplitude of the observed interference fringesdiffers by at least 20%, at least 30% or at least 50% relative to oneanother. For example, fringe 98 has a peak amplitude that is about 50%less than a peak amplitude of a fringe 99.

A low coherence interferometer can be configured to detect aninterference signal is detected over a range of OPD's that is comparableto or greater than the coherence length of the interferometer. Forexample, the range of detected OPD's may be at least 2 times greater orat least 3 times greater than the coherence length. In some embodiments,the coherence length of the detected light is on the order of the heightvariations of features of the object, e.g., on the order of a couple ofmicrons or less but more than a nominal wavelength of the detectedlight.

In general, instrument related contributions to the interference signal,e.g., to the shape and phase of the interference patterns, tend to varyslowly with the topography and position of the object. On the otherhand, interference patterns shift along the scan position axis forinterference signals obtained from object points having differentspatial properties, e.g., different relative heights or differentrelative positions with respect to the interferometer. Accordingly,interference patterns obtained from different object points may havesimilar shapes but are shifted along the scan position axis by an amountrelated to the spatial properties of each point.

With reference to FIG. 2, a template 215 has reduced or eliminatedcontributions related to spatial properties of the object and objectpoints, e.g., object topography (which describes the relative heights ofdifferent points), object position, and object orientationcontributions. Template 215 is representative of the response of aninterferometer to an object and can include instrumental contributionsrepresentative of those appearing in low coherence interference signalsobtained using the interferometer. As discussed above, such instrumentcontributions are similar or the same for interference signals obtainedfrom different objection points. Hence, template 215 can includecontributions from interference signals obtained from different objectpoints. The different points may be arranged about an entire surface tobe analyzed or about one or more subsets of an area to be analyzed.

Interference signals from different object points are processed tocombine, e.g., average, information from a plurality of interferencesignals to prepare the template. The resulting template can have asignificantly higher signal-to-noise level (S/N) than individualinterference signals. In some embodiments, template 215 includescontributions from a plurality of interference signals and has a S/Nthat is at least 10, at least 33, or at least 100 times greater than theindividual interference signals. Applicants have found that interferencesignals can be processed based on such a template to determine one ormore spatial properties of an object.

Referring to FIG. 3, an interference signal 190 is acquired from anobject 191, which includes a substrate 192 and an overlying layer, e.g.,a thin film 193. The substrate and film define an interface 194therebetween. An outer surface of the film 195 defines an interfacebetween the object and its surroundings, e.g., the air, other gas, orvacuum. Interfaces are generally defined by a change in refractive indexbetween portions of an object.

Interference signal 190 includes a first interference pattern 196resulting from interface 194 and a second interference pattern 197resulting from interface 195. First and second interference patterns196,197 are overlapping. For example, maxima of the interferencepatterns 196,197 are separated by an OPD less than the coherence lengthof the interferometer and patterns 196,197 are not separated by a regionof zero intensity. Existing methods for determining spatial propertiesof an object with interfaces that produce overlapping interferencepatterns can yield erroneous results because the overlappinginterference patterns distort one another. Applicants have found thatspatial properties of an object with such interfaces can be determinedbased upon a portion over the overlapping interference patterns. Forexample, a spatial property of interface 195, e.g., a topography of theouter surface of object 191, can be determined based upon a subset 200of interference signal 190. Subset 200 is dominated by contributionsfrom interference pattern 197 (from interface 195) as opposed tocontributions from interference pattern 196 (from interface 194). Aspatial property of interface 194 can be determined based upon a subsetsimilar to subset 200 but located toward the left of the overlappingpatterns.

Referring to FIG. 4, a template 180 is asymmetric, having a shape of atruncated interference pattern, A spatial property of an object havingone or more interfaces, e.g., a spatial property of interface 195 ofobject 190, can be determined based on an asymmetric template 180. Insome embodiments, an interference signal can be processed with anasymmetric template to determine a boundary of a subset of theinterference signal that can be processed to determine a spatialproperty of an interface. For example, interference signal 200 (FIG. 3)can be processed with template 180 (FIG. 4) to determine a boundary 201of subset 200 with interference signal 190 (FIG. 3). The boundary istypically a position along the x-axis of the interference signal, e.g.,a scan position. The subset can be selected based on the boundary. Thesubset can be subjected to further processing to determine a spatialproperty of a particular interface. In some embodiments, an interferencesignal can be processed with an asymmetric template to determine aspatial property of an interface of an object without furtherprocessing.

Low coherence interference signals resulting from objects without thinfilms and from objects without thin films are now discussed in greaterdetail.

Referring back to FIG. 1, low coherence interference signal 150simulates data acquired using a light source having a Gaussiandistribution in wavenumbers with a 100 nm bandwidth in wavelengthcentered about an average of 640-nm, an objective numerical aperture(NA) of 0.3, and a measurement object made of solid silicon nitride(Si₃N₄, index=2.019), which material is partially transparent at 640 nm.

Referring now to FIG. 5, a low coherence interference signal 156simulates data acquired from a measurement object having a silicon (Si,index=3.725−0.029i) substrate coated with 2 μm of Si₃N₄. For clarity,interference signal 156 is simulated without noise. Signal 156 includesa first interference pattern 157 and a second interference pattern 159.First and second interference patterns 157,159 respectively includepeaks 165,167 and fringes 162,164, which decay in accordance withrespective envelopes 158,160. The peaks of the interference patterns arespaced apart along the scan position axis. Interference patterns 157,159respectively correspond to interference resulting from reflections fromthe substrate-film interface and the film-air interface of themeasurement object. Interference patterns 157,159 do not overlap, e.g.,the patterns are spaced apart by a region 169 of essentially zeromodulation intensity. Accordingly, interference signals 157,159 can beprocessed independently of one another to determine spatial propertiesof the object interfaces.

Referring to FIG. 6, a low coherence interference signal 170 simulatesdata acquired from an object having a silicon (Si, index=3.7259−0.029i)substrate coated with a 1.1 μm thick Si₃N₄ thin film. The interferencesignal includes a first interference pattern 172 (resulting from theinterface at the silicon substrate) and a second interference pattern174 (resulting from the outer surface of the Si₃N₄ layer) each patternincluding a plurality of fringes, which decay in accordance withrespective envelopes 181,183. Because of the reduced film thickness (ascompared to FIG. 5), the interference patterns 172,174 overlap creatinga total interference pattern. Conventional data processing would beunable to distinguish between the combined interference effects andwould provide an erroneous interface spatial property, e.g., anincorrect film height, topography, or position.

Referring to FIG. 7, a portion of the interference signal 150 from FIG.1 is plotted with a portion of interference signal 170 of FIG. 6including interference patterns 172 and 174. (Here, for clarity, theinterference pattern of interference signal 150 is shown as discretepoints rather than as points connected to form a line as in FIG. 1. Eachpoint represents a detector intensity observed at a particular scanposition.) Although the presence of the 1.1 μm film alters interferencesignal 170 as compared to interference signal 150, portions ofinterference pattern 174 (film-air interface) and interference pattern151 (resulting from a substrate-air interface) are nearly identical.

In some embodiments discussed herein, a spatial property of a selectedportion of an object including a substrate having one or more layers,e.g., thin films, is determined based on a subset of an interferencesignal. Although interference signals obtained from such objects mayinclude overlapped interference patterns, the signals can include asubset that is relatively undistorted by the overlap. A relativelyundistorted subset of an interference signal can be used to determinespatial properties of the measurement object.

Typically, each subset includes at least a portion of an interferencepattern dominated by interference resulting from the selected portion ofthe measurement object. For example, referring to FIG. 6, a subset 180′of interference signal 170 includes fringes dominated by contributionsfrom interference resulting from the 1.1 μm thick Si₃N₄ thin film-airinterface, as opposed to interference resulting from the underlyingsilicon-film interface, which appears at the left side of theinterference signal. Subset 180′ is asymmetric with respect to scanposition. Properties of subset 180′, including, e.g., the interferencephase, correspond to the Si₃N₄ thin film-air interface with little or noinfluence from the underlying silicon-film interface.

A spatial property of the 1.1 μm thick Si₃N₄ thin film, e.g., a relativeheight of one or more points at its surface, can be determined based onsubset 180′. In some embodiments, the determination gives little or noweight to portions of interference signal 170 outside subset 180′. Forexample, other portions of the interference signal can be suppressed,e.g., set to zero. Subset 180′ can be analyzed to determine a spatialproperty of a portion of the film-air interface of the measurementobject.

In some embodiments, subset 180′ is asymmetric and contains 75% or less,65% or less, or 50% or less of the area under the envelope thatmodulates the intensity of the interference pattern fringes. Forexample, subset 180′ includes only about 50% of the area beneathenvelope 183.

In some embodiments, at least 30%, at least 40%, at least 50%, or atleast 75% of the subset is located to one side of a centroid of aninterference pattern that would be observed in the absence of a secondclosely spaced interface or surface. For example, essentially all ofsubset 180′ is located to the right of the centroid of interferencepattern 151, which is observed in the absence of the film that resultsin the overlapping interference pattern 172 in FIG. 6.

In some embodiments, subset 180′ includes fewer than all of the fringesof the interference signal. For example, referring to FIG. 6,interference signal 170 includes 16 fringes having an intensity at leastas great as threshold intensity 189. Because the interference patternsof FIG. 4 are overlapped (unlike those of FIG. 3) the fringes of bothinterference patterns 172,174 contribute to the total. Subset 180′ mayinclude 50% or fewer of the fringes, 35% or fewer, 25% or fewer, 20% orfewer, or 15% or fewer of the fringes. The threshold intensity may be atleast 2.5%, at least 5%, at least 10%, or at least 20% of a maximumfringe intensity. The threshold intensity may be 30% or less, e.g., 25%or less, 20% or less, or 15% or less of the maximum fringe intensity.

In some embodiments, a width of subset 180′ is determined from aninterference pattern adjacent to an interference pattern correspondingto a surface or interface to be analyzed. For example, interferencepattern 172 results from the substrate-film interface (silicon-Si₃N₄)and interference pattern 174 results from the adjacent, overlyingSi₃N₄-air interface. A dimension, e.g., a width Δ, of the adjacentinterference pattern 172 can be determined from a peak 191 and a scanposition, e.g., scan position 193, at which the amplitude of theadjacent interference pattern has decreased to a selected value, e.g.,25% of the peak, 15% or the peak, 10% of the peak, 5% of the peak, or 2%of the peak. The location of subset 180′ is determined by excluding dataof the interference signal that is located within Δ of peak 191, as byexcluding data to the left of scan position 197.

Alternatively or additionally, a different subset of interference signal170 (e.g., a subset derived from a portion of interference signal 170located to the left in FIG. 6) could be subjected to analysis todetermine a spatial property of the substrate-film interface of themeasurement object.

In some embodiments, a subset of the interference pattern is subjectedto FDA to determine a spatial property of the measurement object. In FDAembodiments, the subset can be transformed to an inverse dimension,e.g., by Fourier transformation.

Typically, the analysis includes determining a rate of change offrequency domain phase with respect to frequency of the transformedsignal. FDA techniques generally are discussed in U.S. Pat. No.5,398,113 entitled “METHOD AND APPARATUS FOR SURFACE TOPOGRAPHYMEASUREMENTS BY SPATIAL-FREQUENCY ANALYSIS OF INTERFERENCE SIGNALS,” thecontents of which are incorporated herein by reference.)

In some embodiments, a subset of the interference pattern is analyzeddirectly in the optical path length difference domain, e.g., in the scandomain without transformation of the signal. Spatial information can bedetermined based on, e.g., the position of a portion of the subset, thespacing of fringes within the subset, or the phase of fringes of thesubset relative to the fringes of a second interference signal.

In some embodiments, at least one template, as determined from theory,experiment, or combination thereof, is used to determine a position orboundary of subset 180′ with respect to an interference signal and/or aspatial property of a portion of a selected portion of a measurementobject, e.g., a topography or position of a substrate-film or film-airinterface. The template may be symmetrical or may be asymmetric as istemplate 180. The template can provide a filter template for matching toa corresponding portion of a measurement interference signal that mayinclude unwanted signals from more than one surface or interface. In thetemplate approach, a matching or data correlation algorithm can be usedto locate, with respect to scan position, a portion of the measurementinterferometry data that corresponds to the interference resulting fromthe selected portion of the measurement object. The location withrespect to scan position of the corresponding portion of theinterferometry data is indicative of the spatial property of theselected portion of the measurement object. The template data may bederived from one or more reference objects, one or more measurementobjects, or combination thereof.

In some embodiments, an interference signal is processed based on atemplate to determine a region of interest within the interferencesignal. For example, a template and an interference signal can becross-correlated to determine a scan position that corresponds to aregion of interest of the interference signal. The portion of theinterference signal to one side of the boundary can be subjected tofurther processing, e.g. by frequency domain analysis (FDA) or in theoptical path length dimension as discussed above.

In some embodiments, an interference signal is processed based on atemplate to determine a scan position that corresponds to a particularinterface, such as the interface between a substrate-thin film or theinterface between a thin film and the environment surrounding theobject, e.g., a thin film-air interface. A photoresist-coated wafer isan example of an object with such interfaces. Once the region ofinterest has been identified, a portion of the interference signal canbe subjected to further analysis, e.g., FDA or phase shifting todetermine an object or object point spatial property, e.g., an objecttopography, position, or orientation. In some embodiments, the analyzedportion of the data is asymmetric and includes only a portion of theinterference pattern resulting from a particular interface. The spatialproperty can be determined accurately even in the presence of nearbyinterfaces, e.g., interfaces separated by 1000 nm or less, 800 nm, orless, 600 nm or less, 500 nm, or less, e.g., 400 nm or less. In someembodiments, one or more spatial properties is determined accuratelyeven in the presence of interfaces separated by 200 nm or more. Forexample, the height and position of one or more points of a thinfilm-air interface can be determined accurately even in the presence ofthe underlying substrate-film interface. In some embodiments, thedistance between the two interfaces is on the order of the coherencelength of the interferometer used to obtain the data, e.g., on the orderof a few microns or less.

In some embodiments, an interference pattern is processed using thetemplate to determine an object spatial property. For example,cross-correlation between the template and an interference signal can beused to determine the height or position of the object and/or one ormore points thereof.

In some embodiments, the template is derived from a first object, e.g.,a reference object without a thin film. One or more interference signalsobtained from a second object, e.g., a measurement object withsubstrate-film and film-air interfaces, are processed based on thetemplate. In some embodiments, a template derived from an object is usedto process the interference signals derived from the same object.

In some embodiments, the spatial property is related to a topography ofthe measurement object, e.g., a height, position, or thickness of alayer covering a substrate. The spatial property may be related to aposition and/or orientation of a portion of the measurement object, suchas a position of a portion of the measurement object relative to anotherobject, e.g., a position of a surface of a layer covering a substratewith respect to a reference portion of a photolithography tool.

Obtaining Interference Signals from an Object

Referring to FIG. 9, an exemplary measurement system 50 for obtaininginterference signals includes an interferometer 51 and automatedcomputer control system 52. The measurement system 50 is operable todetermine one or more spatial properties of a measurement object 53. Insome embodiments, the one or more spatial properties relate to atopography and/or a location of the object 53 with respect to anotherobject, e.g., a portion of system 50. In some embodiments, the otherobject is a reference portion of a photolithography system. In anyevent, system 50 is operable to determine one or more spatial propertiesof objects including one or more at least partially covering layers,e.g., a substrate contacted with a layer of photoresist or solder.

A source 54, which may be a spectrally-broadband source, such as awhite-light lamp, or include a plurality of different wavelengths, e.g.,resulting from a plurality of light emitting diodes, illuminates adiffusing screen 55. As an alternative or in combination with abroadband source, the source 54 can include a narrow band orquasi-monochromatic source, typically having a high numerical aperture.A low coherence interference signal can be obtained using amonochromatic source in combination with a high numerical aperture,e.g., the coherence length may be on the order of a few microns or less.

Lens 56 transmits a collimated beam to a beam-splitting element 57 thattransmits a first portion of the beam to a lens 62 and reference object58. In some embodiments, reference object 58 is optically flat andincludes only a single reflecting surface. For example, reference object58 can be a reference mirror. In some embodiments, reference object 58exhibits a three-dimensional surface topography and/or includes morethan one spaced-apart layer that reflects light. In the followingdiscussion, it is assumed without limitation that reference object 58 isa reference mirror including a single reflective surface.

Beam-splitting element 57 directs a second portion of the beam to a lens60, which focuses the beam onto measurement object 53. Beam-splittingelement 57 combines light reflected from reference mirror 58 and frommeasurement object 53. The combined light is directed to a lens 61,which focuses the combined light to a detector 59. Light reflected frommeasurement object 53 and from mirror 58 interfere at detector 59, whichproduces detector signals indicative of the resultant beam intensity.

Detector 59 typically includes a plurality of detector elements, e.g.,pixels, arranged in at least one and more generally two dimensions. Inthe following discussion, it is assumed without limitation that detector59 includes a two-dimensional array of detector elements, such as a CCDincludes a plurality of pixels. In the embodiment shown, lens 60 andlens 61 focus light reflected from measurement object 53 onto detector59 so that each detector element of detector 59 corresponds to arespective point, e.g., a small region or location of measurement object53. Additionally, lens 62 cooperates with lens 61 to image the referenceobject 58 onto detector 59. Thus, an interference pattern can beobserved at detector 59, even for extended (i.e. spatially incoherent)illumination.

As discussed above, measurement object 53 can include more than onereflective surface such as a substrate including one or more at leastpartially optically transmissive layers. A first reflective surface isdefined by the interface between the outermost optically transmissivelayer and the surrounding atmosphere (or vacuum). Additional reflectivesurfaces are defined by each interface between layers or between layersand the substrate. In such embodiments, the light reflected from themeasurement object 53 can include a contribution, e.g., a separate beam,reflected from each reflective surface or interface. Because eachreflective surface or interface is generally spaced apart along the axisof beam propagation, each separate beam generates a differentinterference pattern when combined with light reflected from themeasurement object 53. The interference pattern observed by detector 59includes the sum of the interference patterns generated by each separatebeam reflected from the measurement object.

System 50 is typically configured to create an optical path lengthdifference (OPD) between light directed to and reflected from referenceobject 58 and light directed to and reflected from measurement object53. In some embodiments, measurement object 53 can be displaced oractuated by an electro-mechanical transducer 63, such as a piezoelectrictransducer (PZT), and associated drive electronics 64 controlled bycomputer 52 so as to effect precise scans along a direction that variesthe OPD of the interferometer 51. In some embodiments, system 50 isconfigured to modify the OPD by moving reference object 58. In someembodiments, system 50 is configured to modify the OPD by an amount atleast as great as height variations in a topography of the object. Insome embodiments, the optical path length is varied by a distance atleast as great as a coherence length of the interferometer, e.g., on theorder of a few microns.

System 50 can acquire a plurality of detector signals as the OPD ismodified, such by scanning a position of measurement object 53. Thedetector signals thus acquired can be stored in digital format as anarray of interference signals, one interference signal acquired fromeach pixel of detector 59, each interference signal representing thevariation in intensity as a function of OPD for a different location ofthe measurement object 53. For example, if the detector 59 includes a128×128 array of pixels and if 64 images are stored during a scan, thenthere will be approximately 16,000 interference signals each 64 datapoints in length. In embodiments using a broadband source 54, theinterference signals may be referred to as scanning white lightinterferometry (SWLI) interference signals, more generally as lowcoherence length scanning interference signals.

After the data has been acquired, the computer 52 can process 67 theinterference signal in accordance with, e.g., methods 100 and 110, andoutput data indicative of a surface topography of the measurement objectVarious aspects of methods 100,110 and data processing 67 are discussednext.

Preparing an Interferometry Template

Referring to FIG. 8, a method 110 for obtaining a template includesobtaining 112 a plurality of typically low coherence referenceinterference signals, e.g., by imaging a plurality of different pointsof an object. The reference interference signals can be generatedtheoretically, determined from reference interference signals obtainedusing a reference object in place of the measurement object, determinedfrom measurement interference signals obtained using the measurementobject itself, or by a combination of such techniques. In any event, theplurality of reference interference signals can be transformed 114 to atransformed dimension to prepare a plurality of transformed interferencesignals, e.g., by Fourier transformation. In step 116, one or morerepresentative transformed interference signals including contributionsfrom more than one of the transformed interferometry sets are prepared.In step 118, a transformed template is prepared. The transformedtemplate can limit or exclude contributions from the topography andposition of the object while retaining contributions from theinterferometer. The transformed template can include contributionsderived from a plurality of the reference interference signals. In someembodiments, the transformed template interferometry data is derivedfrom an average of the transformed interference signals. In step 120,the transformed template is inverse transformed 120 to prepare atemplate, which can be asymmetrical.

Method 110 for obtaining a template is described without limitation inthe context of preparing a template from reference interference signalsobtained 112 from a reference object in place of the measurement object.Method 110, however, may include preparing the template from measurementinterference signals acquired from a measurement object itself. Thereference object typically includes a single reflective surface, e.g.,the object can be free of a transparent coating or film. In someembodiments, the reference object produces SWLI interference signalshaving weak non-linearities, e.g., a silicon carbide flat. The referenceobject may have a three-dimensional topography so that the referenceobject height h_(sys) at different object points. The object is imagedusing an interferometry system with a two-dimensional detector having aplurality of pixels x, e.g., detector 59 of system 50, so that lightdetected from different object points is detected by different detectorpixels. Intensity values are obtained at different scan positions ζ,each position corresponding to a different OPD to obtain a plurality ofinterference signals, typically one for each detector pixel. Thediscussion begins with an exemplary description of SWLI data obtainedusing such a configuration. It should be understood however, that thediscussion is applicable to any low coherence interference signalswhether obtained by, e.g., scanning to vary the optical pathlengthbetween measurement and reference light or by spatially detectingmeasurement and reference light over a plurality of optical path lengthdifferences. The variation in OPD is generally sufficient to modulatethe amplitude of the observed fringes.

Low coherence interference signals can, in some cases, be described viaa pure sinusoidal carrier modulated by an envelope. The modulatedsinusoidal carrier description can apply in cases including e.g. asymmetric source spectrum, non-dispersive optics and a solid surfaceobject. In general, however, these conditions are not met for realinterferometry systems. Accordingly, the following discussion isgeneralized to include an additional, nonlinear OPD-dependent phase termthat absorbs the deviations from a pure sinusoidal carrier. Of course,method 110 can be implemented in terms of other descriptions of lowcoherence interference signals, e.g., the aforementioned sinusoidalcarrier model.

A low coherence interference signal, e.g., a reference interferencesignal I_(sys) (ζ, x), detected by a single detector pixel x, varies asa function of scan position according to:

I _(sys)(ζ,x)=DC _(sys)(x)= . . . AC _(sys)(x)m _(sys) [ζ−h_(sys)(x)]cos{−[ζ−h _(sys)(x)]K ₀+φ_(sys) [ζ−h _(sys)(x)]}

where DC_(sys) is a constant background, AC_(sys) is the amplitude of aninterference signal oscillating at a nominal angular frequency K_(o) andmodulated by an envelope m_(sys) and phase φ_(sys), and h_(sys)(x) isthe height of the object point imaged at pixel x. The signal envelopem_(sys) is related to properties of the light source, the range ofwavelengths detected by the detector, and the numerical aperture of theoptical system. As discussed above, both the envelope m_(sys) and thephase φ_(sys) generally vary slowly with scan position. Interferencepatterns obtained from object locations having different relativepositions, e.g., heights, tend to have similarly shaped envelopefunctions and frequency contents but are shifted along the scan positionaxis by an amount related to the surface height. The followingdiscussion illustrates the determination of a template, which includeslimited or no contributions from the object spatial properties butretains instrumental contributions.

In some embodiments, determining the template includes transforming 114the reference interference signals to a different dimension, e.g., bythe Fourier transformation of each reference interference signal toobtain a respective transformed interference signal q_(sys)(K,x), whereK is the unit of the transformed dimension, e.g., wavenumbers, inversescan position, or data acquisition frequency.

Fourier transformation of an interference signal can be performedaccording to:

$\begin{matrix}{{{q_{sys}\left( {K,x} \right)} = {{FT}\left\{ {I_{sys}\left( {\zeta,x} \right)} \right\}}}{where}} & (1) \\{{{FT}\left\{ {I(\zeta)} \right\}} = {\frac{1}{N}{\int_{- \infty}^{\infty}{{I\left( \hat{\zeta} \right)}{\exp\left( {\; K\; \hat{\zeta}} \right)}\ {\hat{\zeta}}}}}} & (2)\end{matrix}$

and where the normalization integral is

$\begin{matrix}{N = {\int_{- \infty}^{\infty}\ {{\hat{\zeta}}.}}} & (3)\end{matrix}$

Here, the “̂” is used to indicate that {circumflex over (ζ)} is a freevariable of integration in Eqs. (2) and (3). Upon Fourier transformationof the reference interference signals and application of the Fouriershift theorem, the transformed interference signal for each pixel x canbe expressed as:

$\begin{matrix}{{{q_{sys}\left( {K,x} \right)} = {{{\delta (K)}D\; {C_{sys}(x)}} + {\frac{1}{2}A\; {{C_{sys}(x)}\begin{bmatrix}{{G_{sys}^{*\;}\left( {{{- K} - K_{0}},x} \right)} +} \\{G_{sys}\left( {{K - K_{0}},x} \right)}\end{bmatrix}}}}}{where}} & (4) \\{{{G_{sys}\left( {K,x} \right)} = {{\exp \left\lbrack {\; {{Kh}_{sys}(x)}} \right\rbrack}{FT}\left\{ {{m_{sys}(\zeta)}{\exp \left\lbrack {\; {\phi_{sys}(\zeta)}} \right\rbrack}} \right\}}}{and}} & (5) \\{{G_{sys}^{*}\left( {{- K},x} \right)} = {{\exp \left\lbrack {\; {{Kh}_{sys}(x)}} \right\rbrack}{FT}{\left\{ {{m_{sys}(\zeta)}{\exp \left\lbrack {{- }\; {\phi_{sys}(\zeta)}} \right\rbrack}} \right\}.}}} & (6)\end{matrix}$

The K<0 frequency components of the transformed interference signals arethe complex conjugates of the positive-frequency components of thespectrum, with K inverted.

The transformed interference signals may be subjected to a windowingfunction to select a frequency-domain region of interest (ROI), e.g., awindow defined as K_(min) to K_(max). The window may be selected to bedominated by or include only non-DC positive-frequency spectralcomponents of G_(sys) (K−K₀) with meaningful intensity or amplitude withrespect to noise in the ROI. The windowing function may be an apodizingfunction.

Preparing 118 representative transformed interferometry data can includecombining a plurality of the transformed interference signals such as byaveraging with or without weighting, e.g., noise based weighting. Insome embodiments, preparing 118 includes preparing a first transformedinterference signal that includes contributions from the magnitudes ofthe transformed interference signals and a second transformedinterference signal that includes contributions from the phases of thetransformed interference signals. For example, first transformedinterference signal including a combination of the magnitudes of thetransformed interference signals, e.g., the field average of themagnitudes, can be determined as:

$\begin{matrix}{{{\overset{\_}{P_{sys}}\left( {K - K_{0}} \right)} = \frac{\int{{P_{sys}\left( {{K - K_{0}},x} \right)}{x}}}{\int{x}}}{where}} & (7) \\{{P_{sys}\left( {{K - K_{0}},x} \right)} = {A\; {C_{sys}(x)}{{2\; {G_{sys}\left( {{K - K_{0}},x} \right)}}}}} & (8)\end{matrix}$

and a combination of the phases of the transformed interference signals,e.g., the field average of the phases, can be determined as:

$\begin{matrix}{{\overset{\_}{\varphi_{sys}^{\prime}}\left( {K - K_{0}} \right)} = \frac{\int{{\varphi_{sys}^{''}\left( {{K - K_{0}},x} \right)}{x}}}{\int{x}}} & (9)\end{matrix}$

where the measured phase as a function of angular frequency is

φ_(sys)″(K−K _(o) ,x)=connect_(K)[φ_(sys)′″(K−K ₀ ,x)]  (10)

for

φ_(sys)′″(K−K _(o) ,x)=arg[G _(sys)(K−K ₀ ,x)].  (11)

The three primes for the phase data φ_(sys)′″ in the frequency domainindicate that there are multiple 2π uncertainties in the phaseinformation: (1) from angular frequency to angular frequency K, (2) frompixel to pixel, and (3) overall with respect to an absolute reference.The connect_(K) function in Eq. (10) removes one of these primes byconnecting across angular frequencies for each pixel. Examples offunctions for removing such 2π uncertainties are discussed in U.S.application Ser. No. 10/053,106, filed Nov. 2, 2001, titled Heightscanning interferometry method and apparatus including phase gapanalysis and Ghiglia et al., Two Dimensional Phase Unwrapping: Theory,Algorithms, and Software, John Wiley& Sons, Inc., New York, 1998, whichreferences are incorporated herein by reference. The field averaging inEq. (9) removes another prime, leaving only the single prime thatindicates that the overall offset value for the phase is unknown.

A transformed template {tilde over (q)}_(sys) can be prepared 118 as:

{tilde over (q)}_(sys) (K)= P _(sys) (K−K ₀)exp{nonlin_(K)[ φ_(sys)′(K−K₀)]}  (12)

where the function nonlin_(K) returns that portion of the argument thatis nonlinear with respect to angular frequency K, thereby removing thelinear change of phase with angular frequency K. The linear change ofphase with angular frequency is related to the object topography relatedshift of the interference pattern along the scan position axis. Removingthe linear change of frequency can retain certain instrument relatedcontributions to the interference patterns. Thus, the template relatesto a response of the instrument to an object. It should be noted thatthe linear change of phase can be removed prior to combininginterference signals.

The tilde “˜” in Eq. (12) indicates that the transformed template {tildeover (q)}_(sys) contains only the positive nonzero frequencies. As analternative to including only nonzero frequencies, the transformedtemplate can include other frequencies as well.

The transformed template {tilde over (q)}_(sys) can be inversetransformed to prepare a template in the scan domain:

{tilde over (I)}_(sys) (ζ)= m _(sys) (ζ)exp [−iK ₀ ζ=iφ _(sys)(ζ)]  (13)

according to an inverse Fourier transform:

{tilde over (I)}_(sys) (ζ)=FT ⁻¹{ {tilde over (q)}_(sys) (K)}  (14)

FT ⁻¹ {q(K)}=∫_(−∞) ^(∞) q(K)exp(−iKζ)dK  (15)

where, for convenience, the averaged scaling factor AC_(sys) is assigneda value of 1.

The real part of the function Ĩ_(sys) has a readily separable envelopeand phase at each scan position. The envelope is

m _(sys)(ζ)=| {tilde over (I)}_(sys) (ζ)|  (16)

while the phase is

φ_(sys)″(ζ)=connect_(ζ)[ φ_(sys)′″(ζ)]  (17)

for

φ_(sys)′″(ζ)=arg[ {tilde over (I)}_(sys) (ζ)].  (18)

The template may have the same units as an interference signal, e.g.,detector intensity v. scan position. The transformed template may havethe same units as transformed interference signals, e.g., intensity oramplitude v. inverse scan position.

The template may be generally representative of the response of theinterferometer to an object, e.g., a point of an object interface. Forexample, as discussed above, determining the template can includereducing or removing contributions related to, e.g., object-location andsurface height h_(sys), while retaining instrument relatedcontributions, e.g., contributions related to the shape of the envelopeand phase. Contributions from the amplitude of the oscillating andconstant background signals AC_(sys), DC_(sys) may also be reduced orremoved. Either template {tilde over (q)}_(sys) or Ĩ_(sys) can be usedto determine a spatial property of a measurement object whether or notthe template was acquired from a reference object or in another fashion,e.g., from the measurement object itself.

In some embodiments, a portion of the template can be selected for usein determining the spatial property. For example, a windowed portionĨ_(pat) of the template can be selected using a window function:

{tilde over (I)}_(pat)(ζ)=w(ζ) {tilde over (I)}_(sys) (ζ)  (19)

The template window is given by:

$\begin{matrix}{{w(\zeta)} = \left\{ \begin{matrix}1 & {{{for}\mspace{14mu} \zeta_{start}} \leq \zeta \leq \zeta_{stop}} \\0 & {otherwise}\end{matrix} \right.} & (20)\end{matrix}$

An exemplary window is centered about a zero scan position ζ=0 in whichcase an appropriate window may be:

$\begin{matrix}{{\zeta_{start} = {- \frac{\Delta \; \zeta}{2}}}{\zeta_{stop} = {+ \frac{\Delta \; \zeta}{2}}}} & (21)\end{matrix}$

where the window width Δζ may be determined arbitrarily. Alternatively,the end-points of the window function can be defined with respect torelative to a peak amplitude of the envelope, e.g., the window may havea width extending to scan positions corresponding to 10% of the peakamplitude.

In some embodiments, the windowed template is not centered about zeroscan position. In some embodiments, the windowed template isasymmetrical with respect to the interferometry data, e.g., the windowedtemplate may extend to a scan position corresponding to a 10% amplitudeat one side of the template while not extending to a scan positioncorresponding the same relative amplitude on the other side of thetemplate. For example, ζ_(start) can be selected to correspond with thescan position of the peak of the envelope m_(sys) and only template datacorresponding to scan positions on one side of the peak retained. Such atruncated template can be used to determine a region or interest orspatial property of a surface or interface in the presence of more thanone closely spaced layers, e.g., a substrate having a thin film.

Determining a Region of Interest or Spatial Property Based on a Template

Determining a region of interest or a spatial property of a measurementobject can include comparing an interference signal and a template,e.g., by locating a portion of a measurement interference signal thatcorresponds to, e.g., has shape features similar to, a template. Thecomparison can be expressed as a merit function determined fromcross-correlation between the interference signal and the template. Twomethods for comparing an interference signal and a template arediscussed next.

A First Method of Determining a Spatial Property Based on a Template

The following discussion illustrates use of a template Ĩ_(pat) todetermine a region of interest or spatial property of a measurementobject. A plurality of measurement interference signals Ĩ_(ex) areacquired from the measurement object. Each interference signal describesthe detector intensity v. scan position signal for a pixel x as:

I _(ex)(ζ,x)=DC _(ex)(x)+ . . . AC _(ex)(x)m _(ex) [ζ−h_(ex)(x)]cos{−[ζ−h _(ex)(x)]K ₀+φ_(ex) [ζ−h _(ex)(x)]}  (22)

The Fourier transform of each interference signal Ĩ_(ex) can be obtainedas:

$\begin{matrix}{{q_{ex}\left( {K,x} \right)} = {{FT}\left\{ {I_{ex}\left( {\zeta,x} \right)} \right\}}} & (23) \\{{{q_{ex}\left( {K,x} \right)} = {{{\delta (K)}D\; {C_{ex}(x)}} + {\frac{1}{2}A\; {{C_{ex}(x)}\begin{bmatrix}{{G_{ex}^{*}\left( {{{- K} - K_{0}},x} \right)} +} \\{G_{ex}\left( {{K - K_{0}},x} \right)}\end{bmatrix}}}}}{where}} & (24) \\{{G_{ex}(K)} = {{FT}\left\{ {{m_{ex}(\zeta)}{\exp \left\lbrack {\; {\phi_{ex}(\zeta)}} \right\rbrack}} \right\} {{\exp \left\lbrack {\; {{Kh}_{ex}(x)}} \right\rbrack}.}}} & (25)\end{matrix}$

A partial spectrum can be obtained from the positive-frequency portionof each Fourier transformed measurement interference signal:

{tilde over (q)} _(ex)(K)=AC _(ex)(x)G _(ex)(K−K ₀ ,x).  (26)

Each partial spectrum can be inverse transformed as:

Ĩ _(ex)(ζ)=FT ⁻¹ {{tilde over (q)} _(ex)(K)}  (27)

Ĩ _(ex)(ζ,x)=AC _(ex)(x)m _(ex) [ζ−h _(ex)(x)]exp{−[ζ−h _(ex)(x)]K₀+φ_(ex) [ζ−h _(ex)(x)]}  (28)

The real part of each inverse transformed partial spectrum Ĩ_(ex) for apixel x corresponds to the interference signal I_(ex) for the samepixel. Additionally, the phase and envelope of the spectra Ĩ_(ex) arereadily separable by simple operations, e.g. the product of the signalstrength AC_(ex) (x) and the envelope m_(ex) can be determined from themagnitude of the complex function Ĩ_(ex) as:

AC _(ex)(x)m _(ex) [ζ−h _(ex)(x)]=|Ĩ _(ex)(ζ,x)|.  (29)

At least a portion of the envelope m_(pat) of the templateinterferometry data typically has shape features similar to the envelopem_(ex) describing the decay of each spectrum Ĩ_(ex). Differences betweenthe envelopes are typically related to the linear offset h_(ex) of theobject location imaged at each pixel x and the scaling factor AC_(ex)(x). Additionally, differences between the experimental and interferencepattern template phase offsets φ_(ex), φ_(pat) are also related to theheight h_(ex) of the object location imaged at each pixel x. Typically,the differences in the phase offsets are linearly proportional to theheight h_(ex). Accordingly, differences between the envelopes m_(ex),m_(pat) and/or differences between the phase offsets φ_(ex), φ_(pat) canbe used to determine a spatial property of a measurement object. Themethod may include identifying a scan position ζ_(best) for which theshapes of the envelopes m_(ex), m_(pat) and φ_(ex), φ_(pat) are bestmatched. The results of the comparison can be described by a meritfunction. In some embodiments, the merit function is determined bycross-correlating the interference signal and the template. The crosscorrelation can be a complex correlation or a partially-complexcorrelation.

In some embodiments, identifying ζ_(best) is identified using a meritfunction Π derived from the correlation of the template with theinterference signal within a subset of the interferometry data asdefined by the window w:

$\begin{matrix}{{{\prod\left( {\zeta,x} \right)} = \frac{{{\overset{\sim}{I}\left( {\zeta,x} \right)}}^{2}}{{\langle m_{pat}^{2}\rangle}{\langle{{{\overset{\sim}{I}}_{ex}\left( {\zeta,x} \right)}}^{2}\rangle}}}{where}} & (30) \\{{\overset{\sim}{I}\left( {\zeta,x} \right)} = {\frac{1}{N}{\int_{- \infty}^{\infty}{{{\overset{\sim}{I}}_{pat}^{*}\left( \hat{\zeta} \right)}{{\overset{\sim}{I}}_{ex}\left( {{\zeta + \hat{\zeta}},x} \right)}\ {{\hat{\zeta}}.}}}}} & (31)\end{matrix}$

is the complex correlation function and

$\begin{matrix}{{\langle m_{pat}^{2}\rangle} = {\frac{1}{N}{\int_{- \infty}^{\infty}{{{{\overset{\sim}{I}}_{pat}\left( \hat{\zeta} \right)}}^{2}\ {\hat{\zeta}}}}}} & (32) \\{{\langle{{{\overset{\sim}{I}}_{ex}\left( {\zeta,x} \right)}}^{2}\rangle} = {\frac{1}{N}{\int_{- \infty}^{\infty}{{{{\overset{\sim}{I}}_{ex}\left( {{\zeta + \hat{\zeta}},x} \right)}}^{2}{w\left( \hat{\zeta} \right)}\ {\hat{\zeta}}}}}} & (33)\end{matrix}$

are normalizations that make the merit function of independent ofsignal-strength. The normalization can be dependent on a shape of theinterference signal, e.g., upon the moving average of the squaremagnitude of the complex interference signal within the window w. Eq.(30) is the square of the “Pearson's r” familiar from statistics and canalso be derived by a least-squares analysis. Use of the complexconjugate Ĩ_(pat)* of the template cancels the synchronous linear phaseterm K₀ζ and maximizes Π for the case of a match of φ_(ex),φ_(pat). Theabsolute value ∥ of the correlation removes any residual complex phase.

To prevent Π(ζ) from generating false high values or encountering asingularity at low signal levels, a minimum value MinDenom can be addedto the denominator as:

Ĩ _(ex)(ζ)|²

←

Ĩ _(ex)(ζ)|²

+MinDenom·max(

Ĩ _(ex)|²

)  (34)

where the max ( ) function returns the maximum value of the signalstrength |Ĩ_(ex)| over the full scan length ζ, and MinDenom is theminimum relative signal strength that is considered to provide relevantinformation. For example, MinDenom can be set to 5% of the maximumsignal or other value depending upon the level of noise. The correlationmay also be performed by weighting the data to give relative noisy dataless influence in determining the result of the correlation.

The correlation integral Ĩ can be performed in the frequency domainusing the correlation theorem:

Ĩ(ζ)=FT ⁻¹ {{tilde over (q)} _(pat)*(K){tilde over (q)} _(ex)(K)}  (35)

where

FT{Ĩ _(pat)*(ζ,x)}={tilde over (q)} _(pat)*(−K,x)  (36)

and

{tilde over (q)} _(pat)*(K,x)=FT {Ĩ _(pat)(ζ,x)}.  (37)

The merit function Π yields the best match position ζ_(best). Typically,the best match position is a peak of the merit function and the relativeamplitude of the peak is a measure of the quality of the match, rangingfrom zero to one, with one corresponding to a perfect match. The searchfor the best match position can include additional conditions andconstraints to increase robustness. A valid best match position ζ_(best)can be selected to have a signal strength:

|Ĩ _(ex)(ζ_(best))|>GreyLevels·MinMod  (38)

where MinMod is a 0-100% value. A typical MinMod is 10% for a smoothsurface and usually lower for a rough surface. The value of the meritfunction at the best match position can also be required to exceed aselected minimum MinMerit to reduce spurious results:

Π(ζ_(best))>MinMerit  (39)

where MinMerit ranges from 0 to 1, with an exemplary value being about0.3.

For measurement objects lacking closely spaced interfaces or surfaces,the merit function can be searched for the scan position ζ at which Π ismaximized. For measurement objects having closely spaced interfaces orsurfaces, multiple scan positions may satisfy both the MinMod andMinMerit conditions. The scan positions can be identified by, forexample, using a search algorithm that finds the first peak thatsatisfies the MinMerit condition starting at the one end, e.g., the highend of the ζ-coordinate scan. The algorithm further establishesMinRelMod and MinRelMerit conditions, which require that smaller peakshave signals relatively comparable to the highest peaks according tosome percentage value.

An alternative or complementary search routine includes searching themerit function for the first scan position for which Π exceeds a certainthreshold value, even if it is not a peak. This approach may be used foranalysis of data resulting from measurement objects with closely spacedsurfaces or interfaces where, e.g., the presence of an underlyingsubstrate obscures a peak resulting from a film-air interface.

A method for searching Π can include determining a derivative of themerit function, e.g., to locate surface heights.

The best-match position ζ_(best) typically relates to a scan position atwhich the envelopes m_(ex),m_(pat) are aligned. Accordingly, the bestmatch position from cross-correlation can provide a normal resolution orCT-Norm height measurement:

h _(Θ)(x)=ζ_(best)(x)  (40)

where the subscript Θ indicates that the height measurement h_(Θ) (asopposed to the true height h) is based on a coherence approach orfringe-contrast analysis.

As in frequency domain analysis, the normal resolution height profileh_(Θ) may be free of fringe-order uncertainty but have a certain noiselevel. Frequency domain analysis is described in U.S. Pat. No.5,398,113, which is incorporated herein by reference. The noise levelmay be reduced by using the underlying carrier fringes in theinterference pattern.

In correlation template analysis, the argument of the correlationfunction

φ″(ζ)=connect_(ζ){arg[Ĩ(ζ)]}  (41)

has a relative phase value equal to the difference φ_(ex)−φ_(pat). Therelative phase φ″ provides additional, fine-scale information regardingthe template matching. The double prime in Eq. (41) indicates that thereremains a two-fold uncertainty in the fringe order for φ″, both frompixel to pixel within the image and overall with respect to the datum,after connecting across scan positions ζ. The relative phase φ″ evolvesapproximately at a linear rate K₀ with scan position ζ. The relativephase φ″ at the peak correlation position ζ_(best) is the phase gap withrespect to the envelope portion of the interference pattern template:

A″(x)=φ″[ζ_(best)(x)].  (42)

This leads to a higher-resolution phase profile:

Θ″(x)=A″(x)+Θ(x)  (43)

where Θ is the coherence profile in units of phase at the nominalangular frequency K_(o):

Θ(x)=K_(o)h_(Θ)(x).  (44)

The coherence profile Θ(x) in Eq. (44) can be determined using nominalangular frequency K_(o) consistent with the mathematical analysis. Anincorrect K_(o) typically creates interpolation errors. In someembodiments, the template is autocorrelated with itself:

$\begin{matrix}{{{\overset{\sim}{I}}_{auto}(\zeta)} = {\frac{1}{N}{\int_{- \infty}^{\infty}{{{\overset{\sim}{I}}_{pat}^{*}\left( \hat{\zeta} \right)}{{\overset{\sim}{I}}_{pat}\left( {\zeta + \hat{\zeta}} \right)}\ {{\hat{\zeta}}.}}}}} & (45)\end{matrix}$

so that the auto-correlation phase can be searched as a function of scanposition ζ:

φ″_(auto)(ζ)=connect_(ζ) {arg[Ĩ _(auto)(ζ)]}.  (46)

Knowing the increments between scan positions ζ_(step), the nominalfrequency K₀ is given by:

$\begin{matrix}{K_{0} = {\frac{{\phi_{auto}^{''}\left( {\zeta_{best} + {\frac{1}{2}\zeta_{step}}} \right)} - {\phi_{auto}^{''}\left( {\zeta_{best} - {\frac{1}{2}\zeta_{step}}} \right)}}{\zeta_{step}}.}} & (47)\end{matrix}$

The nominal frequency K_(o) can be used to remove the fringe orderuncertainty using a field-connected, approximate phase gap α′ accordingto:

$\begin{matrix}{{\theta^{\prime}(x)} = {{\theta^{''}(x)} - {2\; \pi \; {{Round}\left\lbrack \frac{{A^{''}(x)} - \alpha^{\prime}}{2\; \pi} \right\rbrack}}}} & (48)\end{matrix}$

where Round function returns the nearest integer to its argument. Aphase gap α′ can be determined using, e.g., a sin-cosine averagingtechnique. Exemplary techniques for determining a phase gap aredescribed in U.S. patent application Ser. No. 10/053,106, entitled“HEIGHT SCANNING INTERFEROMETRY METHOD AND APPARATUS INCLUDING PHASE GAPANALYSIS” and filed Nov. 2, 2001, U.S. patent application Ser. No.10/429,175, entitled “PHASE GAP ANALYSIS FOR SCANNING INTERFEROMETRY”and filed May 2, 2003, and “Determination of fringe order in white-lightinterference microscopy,” Appl. Opt. 41(22) 4571 (2002), which documentsare incorporated herein by reference. Upon determining the phase gap,the measurement object height can be determined using the coherenceprofile and nominal frequency K₀ as:

h _(θ)(x)=θ′(x)/K ₀.  (49)

A Second Method of Determining a Spatial Property Based on a Template

The above method for determining a spatial property includedtransformation of the interference signals from the optical path lengthdifference dimension to an inverse dimension, e.g., inverse scandimension. In some embodiments, interference signals are processed inthe scan dimension, e.g., without transformation to an inversedimension. The processing can include cross-correlating the interferencesignal and a template in the scan dimension. For example, the processingcan include subjecting each interference signal and a template to apartially complex correlation as follows:

$\begin{matrix}{{\underset{\sim}{I}\left( {\zeta,x} \right)} = {\frac{2}{N}{\int_{- \infty}^{\infty}{{{\overset{\sim}{I}}_{pat}^{*}\left( \hat{\zeta} \right)}{I_{ex}\left( {{\zeta + \hat{\zeta}},x} \right)}\ {{\hat{\zeta}}.}}}}} & (50)\end{matrix}$

The properties of the template can be selected to enhance thesensitivity of the partially complex correlation to best match positionsbetween the interference signal and template. Writing the interferencesignal as:

$\begin{matrix}{{{I_{ex}\left( {\zeta,x} \right)} = {{{DC}_{ex}(x)} + {\frac{1}{2}\left\lbrack {{{\overset{\sim}{I}}_{ex}\left( {\zeta,x} \right)} + {{\overset{\sim}{I}}_{ex}^{*}\left( {\zeta,x} \right)}} \right\rbrack}}},} & (51)\end{matrix}$

the partially complex correlation can be expressed as a sum of twoparts:

{tilde under (I)}(ζ,x)=Ĩ(ζ,x)+err(ζ,x)  (52)

where the undesired err term is given by:

$\begin{matrix}{{{err}\left( {\zeta,x} \right)} = {{\frac{2\; {{DC}_{ex}(x)}}{N}{\int_{- \infty}^{\infty}{{{\overset{\sim}{I}}_{pat}^{*}\left( \hat{\zeta} \right)}\ {\hat{\zeta}}}}} + {\frac{1}{N}{\int_{- \infty}^{\infty}{{{\overset{\sim}{I}}_{pat}^{*}\left( \hat{\zeta} \right)}{{\overset{\sim}{I}}_{ex}^{*}\left( {{\zeta + \hat{\zeta}},x} \right)}\ {{\hat{\zeta}}.}}}}}} & (53)\end{matrix}$

Properties of the template Ĩ_(pat) can be selected to make the errsmall, e.g., drive the term toward zero so that the result of thecorrelation is determined by the first term on the left of Eq. 53. Forexample, the Fourier transform {tilde over (q)}_(pat) of such a templateĨ_(pat) may have low or negligible values outside of thepositive-frequency neighborhood of the expected frequencies of theinterference signal. In some embodiments, the average or DC value of thecomplex portion of the template Ĩ_(pat)* are zero, and the correlationof Ĩ_(pat)* and (the mathematically hypothetical) Ĩ_(ex)* is also zero.For example, the template Ĩ_(pat) can have zero negative frequencycomponents.

A template Ĩ_(pat) consisting essentially of strong, non-DCpositive-frequency spectral components in the range K_(min). K_(max) canbe prepared by subjecting the transformed template prepared as discussedabove to an apodizing window function. In some embodiments, theapodizing windowing function includes a raised cosine or von-Hann windowinstead of a window with a sharper cut-off in the frequency domain:

$\begin{matrix}{{{ROI}(K)} = {0.5 + {0.5\; {{\cos \left\lbrack {2\; {\pi \left( \frac{K - {K\; 0}}{\Delta \; K} \right)}} \right\rbrack}.}}}} & (54)\end{matrix}$

The apodized windowing function reduces ringing when generating Ĩ_(sys)and makes it easier to contain all of the important information aboutthe interference pattern within a limited scan length. Note that toaccommodate the apodization, the total frequency-domain range ΔK may belarger than a square or rectangular windowing function, e.g., twice aswide in the frequency domain.

A scan domain windowing function w having reduced sharpness as comparedto a square or rectangular window can also be used:

$\begin{matrix}{{w(\zeta)} = {0.5 + {0.5\; {\cos \left( \frac{2\; \pi \; \zeta}{\Delta \; \zeta} \right)}}}} & (55)\end{matrix}$

The interference pattern template is given by the product of thetemplate and the scan domain windowing function:

{tilde over (I)}_(pat)(ζ)=w(ζ) {tilde over (I)}_(sys)   (56)

where Ĩ_(sys) includes contributions from a plurality of interferencesignals. An asymmetric window can be prepared by shifting w to the rightby an amount Δζ/4. Typically, however, the window is set about the ζ=0position.

A final in-line pattern template Ĩ_(pat.inl) can be prepared byretaining only nonzero values and noting the starting offset ζ_(offs).The function Ĩ_(pat.inl) is used as a kernel in a discretepartially-complex correlation:

$\begin{matrix}{{{\underset{\sim}{I}\left( {\zeta,x} \right)} = {\frac{2}{N}{\int_{- \infty}^{\infty}{{{\overset{\sim}{I}}_{{pat}.{inl}}^{*}\left( \hat{\zeta} \right)}{I_{ex}\left( {{\zeta + \hat{\zeta} + \zeta_{offs}},x} \right)}\ {\hat{\zeta}}}}}},} & (57)\end{matrix}$

where the optional offset ζ_(offs) preserves the zero position so thatit coincides with that of a Fourier transformation implementation.

The number of frames or buckets for the convolution kernel Ĩ_(pat.inl)varies depending on the coherence length of the instrument, the scanrate (nm/frame) and the pattern threshold parameter. For example, withthe pattern threshold set to 20%, the Δζ for the window w spans thewidth of the system characterization signal envelope to the 20% oflevels on either side of maximum. For a 100-nm bandwidth, 560-nm centerwavelength and an 80-nm/frame scan rate, the kernel Ĩ_(pat.inl) spansapproximately 23 buckets. Upon increasing the pattern threshold to 40%,Ĩ_(pat.inl) decreases to 17 buckets.

A merit function can be determined as follows:

$\begin{matrix}{{\Pi_{inline}\left( {\zeta,x} \right)} = \frac{{{\underset{\sim}{I}\left( {\zeta,x} \right)}}^{2}}{{\langle m_{pat}^{2}\rangle}{\langle m_{ex}^{2}\rangle}}} & (58)\end{matrix}$

where {tilde under (I)} is the in-line, partially complex correlationdefined by Eq. (57). The normalization can be determined from aselected, arbitrary single strength, e.g., a signal just undersaturation. For example, such a signal may be one-half the number ofdigital grey levels in the detector:

$\begin{matrix}{{\langle m_{ex}^{2}\rangle} = {\frac{1}{N}\left( \frac{greylevels}{2} \right)^{2}{\int_{- \infty}^{\infty}{{w\left( \hat{\zeta} \right)}\ {{\hat{\zeta}}.}}}}} & (59)\end{matrix}$

The merit function oscillates with interference signal strength, even ifthe quality of the pattern match does not change. In some embodiments,determination of the merit function operates similarly to theapplication of a PSI algorithm (defined by the conjugate kernelĨ_(pat.inl)*) proceeding step by step through an interference signalI_(ex). This can be illustrated by rewriting the partially complexcorrelation of Eq. (57) in terms of discrete data:

$\begin{matrix}{{\underset{\sim}{I}}_{z} = {{\sum\limits_{\hat{z}}{c_{\hat{z}}\left( I_{ex} \right)}_{\hat{z}}} + {i{\sum\limits_{\hat{z}}{s_{\hat{z}}\left( I_{ex} \right)}_{\hat{z}}}}}} & (60)\end{matrix}$

with algorithm coefficients defined as:

s _({circumflex over (z)}) =−Im{(Ĩ_(pat.inl))_({circumflex over (z)})}  (61)

c _({circumflex over (z)}) =Re{(Ĩ_(pat.inl))_({circumflex over (z)})}.  (62)

Therefore:

$\begin{matrix}{{{{\underset{\sim}{I}}_{z}} = \sqrt{\left\lbrack {\sum\limits_{\hat{z}}{s_{\hat{z}}\left( I_{ex} \right)}_{\hat{z}}} \right\rbrack^{2} + \left\lbrack {\sum\limits_{\hat{z}}{c_{\hat{z}}\left( I_{ex} \right)}_{\hat{z}}} \right\rbrack^{2}}}{and}} & (63) \\{{\arg \left\{ {\underset{\sim}{I}}_{z} \right\}} = {{arc}\; \tan {\left\{ \frac{\sum\limits_{\hat{z}}{s_{\hat{z}}\left( I_{ex} \right)}_{\hat{z}}}{\sum\limits_{\hat{z}}{c_{\hat{z}}\left( I_{ex} \right)}_{\hat{z}}} \right\}.}}} & (64)\end{matrix}$

The merit function based on the partially complex correlation can besearched to identify locations that correspond to a region of interest,e.g., to an interface of an object. For example, to determine a scanposition corresponding to an outer surface of an object, the meritfunction can be searched for the first peak from the right (assumingthat scan positions to the right correspond to increased distances fromthe object). To determine a scan position corresponding to an outersurface of a film, the merit function can be searched for the first peakfrom the right assuming that at least two merit peaks are present. Filmthickness can be determined from the scan positions corresponding to thestrongest two peaks of the merit function.

In some embodiments, the merit function approximates the square of thenormalized signal strength. Accordingly, the square root of the meritfunction can be searched for peaks:

${{m_{ex}\left( {\zeta,x} \right)} \approx {\frac{greylevels}{2}\sqrt{\Pi_{inline}\left( {\zeta,x} \right)}}},$

The merit function can be reduced, with all values below MinMod² set tozero. The reduced merit function is then searched for peaks. Once a peakis located, more precise information can be obtained by interpolationbetween neighboring discrete scan positions using the originalΠ_(inline) merit function.

The merit function Π differs from a simple signal strength or envelopecalculation in that it evaluates how well the experimental signalmatches the interference pattern template Ĩ_(pal) within the window w.For this reason, it requires normalization to the signal magnitude.

Processing and Processor Code

Any of the computer analysis methods described above can be implementedin hardware or software, or a combination of both. The methods can beimplemented in computer programs using standard programming techniquesfollowing the method and figures described herein. Program code isapplied to input data to perform the functions described herein andgenerate output information. The output information is applied to one ormore output devices such as a display monitor. Each program may beimplemented in a high level procedural or object oriented programminglanguage to communicate with a computer system. However, the programscan be implemented in assembly or machine language, if desired. In anycase, the language can be a compiled or interpreted language. Moreover,the program can run on dedicated integrated circuits preprogrammed forthat purpose.

Each such computer program is preferably stored on a storage medium ordevice (e.g., ROM or magnetic diskette) readable by a general or specialpurpose programmable computer, for configuring and operating thecomputer when the storage media or device is read by the computer toperform the procedures described herein. The computer program can alsoreside in cache or main memory during program execution. The analysismethod can also be implemented as a computer-readable storage medium,configured with a computer program, where the storage medium soconfigured causes a computer to operate in a specific and predefinedmanner to perform the functions described herein.

Exemplary Applications

The low coherence interferometry methods and systems described above mayused for any of the following surface analysis problems: simple thinfilms; multilayer thin films; sharp edges and surface features thatdiffract or otherwise generate complex interference effects; unresolvedsurface roughness; unresolved surface features, for example, asub-wavelength width groove on an otherwise smooth surface; dissimilarmaterials; polarization-dependent properties of the surface; anddeflections, vibrations or motions of the surface or deformable surfacefeatures that result in incident-angle dependent perturbations of theinterference phenomenon. For the case of thin films, the variableparameter of interest may be the film thickness, the refractive index ofthe film, the refractive index of the substrate, or some combinationthereof. Exemplary applications including objects and devices exhibitsuch features are discussed next.

Photolithography

In many microelectronics applications, photolithography is used topattern a layer of photoresist overlying a substrate, e.g., a siliconwafer. Referring to FIGS. 10 a and 10 b, an object 30 includes asubstrate, e.g., a wafer, 32 and an overlying layer, e.g., photoresistlayer 34. Object 30 includes a plurality of interfaces as occur betweenmaterials of different refractive index. For example, anobject-surroundings interface 38 is defined where an outer surface 39 ofphotoresist layer 34 contacts the environment surrounding object 30,e.g., liquid, air, other gas, or vacuum. A substrate-layer interface 36is defined between a surface 35 of wafer 32 and a bottom surface 37 ofphotoresist layer 34. Surface 35 of the wafer may include a plurality ofpatterned features 29. Some of these features have the same height asadjacent portions of the substrate but a different refractive index.Other features may extend upward or downward relative to adjacentportions of the substrate. Accordingly, interface 36 may exhibit acomplex, varying topography underlying the outer surface of thephotoresist.

A photolithography apparatus images a pattern onto the object. Forexample, the pattern may correspond with elements of an electroniccircuit (or the negative of the circuit). After imaging, portions of thephotoresist are removed revealing the substrate underlying the removedphotoresist. The revealed substrate can be etched, covered withdeposited material, or otherwise modified. Remaining photoresistprotects other portions of the substrate from such modification.

To increase manufacturing efficiencies, more than one device issometimes prepared from a single wafer. The devices may be the same ordifferent. Each device requires that a subset of the wafer be imagedwith a pattern. In some cases, the pattern is sequentially imaged ontodifferent subsets. Sequential imaging can be performed for severalreasons. Optical aberrations can prevent achieving adequate patternfocus quality over larger areas of the wafer. Even in the absence ofoptical aberrations, the spatial properties of the wafer and photoresistmay also prevent achieving adequate pattern focus over large areas ofthe wafer. Aspects of the relationship between the spatial properties ofthe wafer/resist and focus quality are discussed next.

Referring to back to FIG. 10 b, object 30 is shown with a number Nsubsets 40 _(i), each smaller than a total area 41 the object to beimaged. Within each subset 40 _(i), spatial property variations, e.g.,height and slope variations of the wafer or photoresist, are typicallysmaller than when taken over the total area 41. Nonetheless, the waferor photoresist of different subsets 40 _(i) typically have differentheights and slopes. For example, layer 34 exhibits thicknesses Δt₁ andΔt₂, which vary the height and slope of surface 39 (FIG. 10 a). Thus,each subset of the object may have a different spatial relationship withthe photolithography imager. The quality of focus is related to thespatial relationship, e.g., the distance between the object and thephotolithography imager. Bringing different subsets of the object intoproper focus may require relative repositioning of the object andimager. Because of the object height and slope variations, proper subsetfocus cannot be achieved solely by determining the position andorientation of the object with respect to a portion of the object thatis remote to the imaged subset, e.g., a side 43 of the object.

Proper focus can be achieved by determining a spatial property of anobject within a subset of the object to be imaged (or otherwiseprocessed). Once the position of the subset has been determined, theobject (and/or a portion of the photolithography imager) can be moved,e.g., translated, rotated, and/or tilted, to modify the position of thesubset with respect to a reference, e.g., a portion of thephotolithography imager. The determination and movement (if necessary)can be repeated for each subset to be imaged.

The determination of the spatial property of the subset can includedetermining a position and/or height of one or more points of an outersurface of a thin layer of the object, the one or more points lyingwithin the subset of the object to be imaged. For example, the positionand orientation of the outer surface 39 of subset 40 ₂ (FIG. 1 a) can bedetermined based upon the positions of points 42 ₁-42 ₃ within thesubset. The determination of the spatial property of the subset to beimaged can include using an interferometer to illuminate the subset withlight and detecting an interference signal including light reflectedfrom the illuminated subset. In some embodiments, a plurality of subsetsare simultaneously imaged with light to obtain a plurality ofinterference signals. Each interference signal is indicative of one ormore spatial properties of a subset. Thus, the interference signals canbe used to prepare an image indicative of the topography of the objectover a plurality of the subsets. During photolithography of the subsets,the wafer is positioned based upon the topography of the individualsubsets as determined from the plurality of interference signals. Hence,each subset can be positioned for optimum focus with respect to thephotolithography apparatus.

Detecting an interference signal from each subset of an object to beimaged can include detecting light reflected from the subset andreference light over an OPD range that is at least as large as acoherence length of the detected light. For example, the light may bedetected at least over its coherence length. In some embodiments, theinterferometer is configured so that the light reflected from theilluminated subset is dominated by light reflected from either an outerinterface (such as outer surface 39) or an inner interface (such asinterface 36). In some embodiments, a spatial property of an object isdetermined based on only a portion of the interference signal. Forexample, if the interference signal includes two or more overlappinginterference patterns, a spatial property of the object can bedetermined based upon a portion of one of the interference patterns thatis dominated by contributions from a single interface of the object.

Copper Interconnect Structures And Chemical Mechanical Polishing

It is becoming common among chip makers to use the so-called ‘dualdamascene copper’ process to fabricate electrical interconnects betweendifferent parts of a chip. This is an example of a process which may beeffectively characterized using a suitable surface topography system.The dual damascene process may be considered to have six parts: (1) aninterlayer dielectric (ILD) deposition, in which a layer of dielectricmaterial (such as a polymer, or glass) is deposited onto the surface ofa wafer (containing a plurality of individual chips); (2) chemicalmechanical polishing (CMP), in which the dielectric layer is polished soas to create a smooth surface, suitable for precision opticallithography, (3) a combination of lithographic patterning and reactiveion etching steps, in which a complex network is created comprisingnarrow trenches running parallel to the wafer surface and small viasrunning from the bottom of the trenches to a lower (previously defined)electrically conducting layer, (4) a combination of metal depositionsteps which result in the deposition of copper trenches and vias, (5) adielectric deposition step in which a dielectric is applied over thecopper trenches and vias, and (6) a final CMP step in which the excesscopper is removed, leaving a network of copper filled trenches (andpossibly vias) surrounded by dielectric material.

Referring to FIG. 11 a, a device 500 is exemplary of the a filmstructure resulting from the deposition of a dielectric 504 over copperfeatures 502 deposited on a substrate 501. The dielectric 504 has anon-uniform outer surface 506 exhibiting height variations therealong.Interference signals obtained from device 500 can include interferencepatterns resulting from surface 506, an interface 508 between copperfeatures 502 and dielectric 504, and an interface 510 between substrate501 and dielectric 504. The device 500 may include a plurality of otherfeatures that also generate interference patterns.

Referring to FIG. 11 b, a device 500′ illustrates the state of device500 after the final CMP step. The upper surface 506 has been planarizedto a surface 506′, and interface 508 may now be exposed to thesurroundings. Interface 510 at the substrate surface remains intact.Device performance and uniformity depends critically on monitoring theplanarization of surface 504. It is important to appreciate that thepolishing rate, and therefore the remaining copper (and dielectric)thickness after polishing, depends strongly and in a complex manner onthe polishing conditions (such as the pad pressure and polishing slurrycomposition), as well as on the local detailed arrangement (i.e.,orientation, proximity and shape) of copper and surrounding dielectricregions. Hence, portions of surface 506 over copper elements 502 mayetch at different rates than other portions of surface 506.Additionally, once interface 508 of copper elements 502 is exposed, thedielectric and copper elements may exhibit different etch rates.

This ‘position dependent polishing rate’ is known to give rise tovariable surface topography on many lateral length scales. For example,it may mean that chips located closer to the edge of a wafer onaggregate are polished more rapidly than those located close to thecenter, creating copper regions which are thinner than desired near theedges, and thicker than desired at the center. This is an example of a‘wafer scale’ process nonuniformity—i.e., one occurring on length scalecomparable to the wafer diameter. It is also known that regions whichhave a high density of copper trenches polish at a higher rate thannearby regions with low copper line densities. This leads to aphenomenon known as ‘CMP induced erosion’ in the high copper densityregions. This is an example of a ‘chip scale’ processnon-uniformity—i.e., one occurring on a length scale comparable to (andsometimes much less than) the linear dimensions of a single chip.Another type of chip scale nonuniformity, known as ‘dishing’, occurswithin single copper filled trench regions (which tend to polish at ahigher rate than the surrounding dielectric material). For trenchesgreater than a few microns in width dishing may become severe with theresult that affected lines later exhibit excessive electricalresistance, leading to a chip failure.

CMP induced wafer and chip scale process nonuniformities are inherentlydifficult to predict, and they are subject to change over time asconditions within the CMP processing system evolve. To effectivelymonitor, and suitably adjust the process conditions for the purpose ofensuring that any nonuniformities remain within acceptable limits, it isimportant for process engineers to make frequent non-contact surfacetopography measurements on chips at a large number and wide variety oflocations. This is possible using embodiments of the interferometrymethods and systems described above.

In some embodiments one or more spatial properties, e.g., the topographyof surface 506 and/or the thickness of dielectric 504, are monitored byobtaining low coherence interference signals from the structure beforeand/or during CMP. Based on the spatial properties, the polishingconditions can be changed to achieve the desired planar surface 506′.For example, the pad pressure, pad pressure distribution, polishingagent characteristics, solvent composition and flow, and otherconditions can be determined based on the spatial properties. After someperiod of polishing, the spatial property can again be determined andthe polishing conditions changed as needed. The topography and/orthickness is also indicative of the end-point at which, e.g., surface504′ is achieved. Thus, the low coherence interference signals can beused to avoid depressions caused by over polishing different regions ofthe object. The low coherence interference methods and systems areadvantageous in this respect because spatial properties of the device,e.g., the relative heights of the surface of the dielectric (a) overcopper elements 502 and (b) over substrate surface 510 but adjacentcopper elements 502 can be determined even in the presence of themultiple interfaces.

Solder Bump Processing

Referring to FIGS. 12 a and 12 b, a structure 550 is exemplary of astructure produced during solder bump processing. Structure 550 includesa substrate 551, regions 502 non-wettable by solder, and a region 503wettable by solder. Regions 502 have an outer surface 507. Region 503has an outer surface 509. Accordingly, an interface 505 is formedbetween regions 502 and substrate 501.

During processing a mass of solder 504 is positioned in contact withwettable region 503. Upon flowing the solder, the solder forms a securecontact with the wettable region 503. Adjacent non-wettable regions 502act like a dam preventing the flowed solder from undesirable migrationabout the structure. It is desirable to know spatial properties of thestructure including the relative heights of surfaces 507, 509 and thedimensions of solder 504 relative to surface 502. As can be determinedfrom other discussions herein, structure 550 includes a plurality ofinterfaces that may each result in an interference pattern. Overlapbetween the interference patterns prevents accurate determinate of thespatial properties using known interference techniques. Application ofthe systems and methods discussed herein allow the spatial properties tobe determined.

Spatial properties determined from structure 550 can be used to changemanufacturing conditions, such as deposition times for layers 502,503and the amount of solder 504 used per area of region 503. Additionally,heating conditions used to flow the solder can also be changed based onthe spatial properties to achieve adequate flow and or prevent migrationof the solder.

Liquid Crystal Displays

Referring to FIG. 13, a passive matrix LCD 450 is composed of severallayers. The main parts are two glass plates 452,453 connected by seals454. A polarizer 456 is applied to the front glass plate 453 in order topolarize incoming light in a single direction. The polarized lightpasses through the front glass plate 453. An Indium Tin Oxide (ITO)layer 458 is used as an electrode. A passivation layer 460, sometimescalled hard coat layer, based on SiOx is coated over the ITO 458 toelectrically insulate the surface. Polyimide 462 is printed over thepassivation layer 460 to align the liquid crystal fluid 464. The liquidcrystal fluid is sensitive to electric fields and changes orientationwhen an electric field is applied. The liquid crystal is also opticallyactive and rotates the polarization direction of the incoming light. Thecell gap Δg, i.e., thickness of the liquid crystal layer 464, isdetermined by spacers 466, which keep the two glass plates 452,453 at afixed distance. When there is no electric potential from the front plate453 to the rear plate 452, the polarized light is rotated 90° as itpasses through the liquid crystal layer 464. When an electric potentialis applied from one plate to the other plate the light is not rotated.After the light has passed through the liquid crystal layer 464, itpasses through another polyimide layer 468, another hard coat layer 470,a rear ITO electrode 472, and the rear glass plate 452. Upon reaching arear polarizer 474, the light either transmitted through or absorbed,depending on whether or not it has been rotated 90°. The cell 450 mayinclude filters 476 or other colorizing elements to provide a colordisplay.

The cell gap Δg determines to a great extent the optoelectricalproperties of the LCD, e.g., the contrast ratio and brightness. Cell gapcontrol during manufacturing is critical to obtaining uniform, qualitydisplays. The actual cell gap may differ from the dimensions of spacers466 because, during assembly, pressure or vacuum is applied to introducethe liquid crystal medium, seals 454 cure and may change dimensions, andthe added liquid crystal medium generates capillary forces betweenplates 452,453. Both before and after adding the liquid crystal medium464, surfaces 480,482 of plates 452,453 reflect light that results in aninterference pattern indicative of the cell gap Δg. The low coherencenature of the interference signal either itself or in combination withthe described interference signal processing techniques can be used tomonitor properties of the cell including the cell gap Δg duringmanufacture even in the presence of interfaces formed by other layers ofthe cell.

An exemplary method can include obtaining a low coherence interferencesignal including interference patterns indicative of the cell gap Δgprior to adding layer 464. The cell gap (or other spatial property ofthe cell) is determined from the interference patterns and can becompared to a specified value. Manufacturing conditions, e.g., apressure or vacuum applied to plates 452,453 can be changed to modifythe cell gap Δg if a difference between the specified value and thedetermined cell gap exceeds tolerances. This process can be repeateduntil achieving the desired cell gap. Liquid crystal medium is thenintroduced into the cell. The amount of liquid crystal medium to beadded can be determined from the measured spatial property of the cell.This can avoid over- or underfilling the cell. The filling process canalso be monitored by observing interference signals from the surfaces480,482. Once the cell has been filed, additional low coherenceinterference patterns are obtained to monitor the cell gap Δg (or otherspatial property). Again, the manufacturing conditions can be changed sothat the cell gap is maintained or brought within tolerances.

Laser Scribing and Cutting

Lasers can be used to scribe objects in preparation for separatingdifferent, concurrently manufactured structures, e.g., microelectronicsstructures. The quality of separation is related to the scribingconditions, e.g., laser focus size, laser power, translation rate of theobject, and scribe depth. Because the density of features of thestructure may be large, the scribe lines may be adjacent thin film orlayers of the structures. Interfaces associated with the thin film orlayers may create interference patterns that appear when interferometryis used to determine the scribe depth. The methods and systems describedherein can be used to determine the scribe depth even in the presence ofsuch adjacent films or layers.

An exemplary method can include scribing one or more electronicstructures and separating the structures along the scribe lines. Beforeand/or after separation, low coherence interference signals can be usedto determine the depth of scribe. Other scribing conditions are known,e.g., laser spot size, laser power, translation rate. The scribe depthcan be determined from the interference signals. The quality ofseparation as a function of the scribing conditions, including thescribe depth, can be determined by evaluating the separated structures.Based on such determinations, the scribing conditions necessary toachieve a desired separation quality can be determined. During continuedmanufacturing, low coherence interference signals can be obtained fromscribed regions to monitor the process. Scribing conditions can bechanged to maintain or bring the scribe properties within tolerances.

Examples

Determining a spatial property of a measurement object is furtherdescribed the context of the following non-limiting examples.

1. Determining a Spatial Property of a Single-Surface Measurement Object

Referring to back to FIG. 1, interference signal 150 is but one of atotal 101 interference signals representing a linear trace across theobject surface. For convenience, the remaining 100 interference signalsare not shown. The silicon dioxide object surface has an approximatelyspherical profile with PV=600 nm. The irradiation wavelength is 550 nmwith a bandwidth of 100 nm. The bandwidth is Gaussian in wavenumber. Thenumerical aperture is 0.01 for normal incidence, collimated light. Eachinterference signal has a full scale digital resolution of 256 greyscale steps. The average signal strength is 20 grey levels amplitude ACabove 65 grey levels DC. The signals have random noise having a standarddeviation of 2 grey levels.

The 101 interference signals are transformed to an inverse domain usingthe Fourier transform. Referring to FIG. 14 a, the magnitude of theFourier transform 205 of interference signal 200 has a peak located atabout 3.7 cycles per micron of PLD. The transformed interference signalsare used to prepare a transformed template including contributions fromall of the transformed interference signals. A region of interest 202 isshown as centered about the peak.

Referring to FIG. 14 b, a transformed interference signal 204 thatincludes contributions from the 101 transformed interference signals hasa substantially higher signal to noise ratio (S/N) than each transformedinterference signals as can be seen upon comparing FIGS. 10 a and 10 b.

Referring to FIG. 15, a template 210 includes a plurality of fringesthat decay according to an envelope 212. Template 210 has asubstantially higher S/N than each interference signal as can be seenupon comparing FIGS. 1 and 11. Template 212 further differs from the 101measurement interference signals (e.g., set 150) in that contributionsfrom the object surface height, phase differences between differentinterference signals, and the DC bias have been removed from thetemplate.

Referring back to FIG. 2, a windowed template 215 represents the productof template 212 and a window function 217, with values of template 212outside of the window function set to zero.

Referring to FIG. 16, a function 220 is given by the product of theamplitude of the AC portion of the oscillating signal and the envelopedescribing the decay of the fringes (AC_(ex)m_(ex)) of interferencesignal 200 of FIG. 1.

Referring to FIG. 17, a merit function 230 obtained by complexcross-correlation of the windowed template of FIG. 2 and theinterference signal 150 of FIG. 1.

Referring to FIG. 18, the surface height of the measurement object isshown as a function of lateral position across the object surface asdetermined from the template 215 and 101 interference signals using acoherence profile 240 h_(Θ) approach and phase profile 242 h_(Θ)approach.

2. Determining a Spatial Property of a Measurement Object with Thin Film

Referring to FIG. 19, a reference interference signal 300 is one of 101reference interference signals obtained as from the same referenceobject used in Example 1.

Referring to FIGS. 20 a and 20 b, the 101 reference interference signalsare transformed to an inverse domain by Fourier transformation. Themagnitude of the Fourier transform 305 of interference signal 300 has apeak located at about 3.7 cycles per micron of OPD. A transformedinterference signal 304 that includes contributions from the magnitudesof the 101 transformed interference signals has a substantially highersignal to noise ratio (S/N) than each transformed interference signalsas can be seen upon comparing FIGS. 20 a and 20 b.

A template (not shown) having characteristics similar to template 212 ofFIG. 15 is prepared from the 101 transformed interference signals.Referring to FIG. 21, an asymmetric windowed template 315 represents theproduct of the template and a window function 317, with values of thetemplate outside of the window function 317 set to zero. The leftboundary of the template corresponds to the peak of the envelope thatdetermines the decay of the fringes.

Referring to FIG. 22, a measurement interference signal 400 is obtainedfrom a measurement object including silicon dioxide film on a siliconsubstrate (SiO₂ on Si). The substrate of the object is the same as thatused to acquire the reference interference signals, i.e., the substrateis the same as the measurement object used in Example 1. The filmthickness ranges from 900 nm at edges to 1500 nm in the middle (at pixel50). A linear strip of 101 measurement interference signals are acquiredunder the same conditions as for Example 1.

As seen in FIG. 22, interference signal 400 includes a first and secondinterference patterns 402,404 respectively resulting from thesubstrate-film interface and the film-air interface. The interferencepatterns are partially overlapped.

Referring to FIG. 23, a function 420 is given by the product of theamplitude of the AC portion of the oscillating signal and the envelopesdescribing the decays of the fringes of the first and secondinterference patterns 402,404 interference signals 400 of FIG. 22.

Referring to FIG. 24, a merit function 425 includes first and secondpeaks 427,429 as a function of scan position. Merit function 425 isobtained by cross-correlating the asymmetric template of FIG. 22 withthe interference signal of FIG. 23. Each point of the cross-correlationis normalized as discussed with respect to Eq. 30. A line 431 indicatesthe location of the first peak position ζ_(best) with respect to scanposition. The first peak position ζ_(best) indicates the location of thefilm-air interface.

Referring to FIG. 25 height profiles for the film-air interface areshown. A profile 440 h_(Θ) (coherence) is determined from the using acoherence approach and a height profile 442 h_(θ) (phase) is determinedusing a phase approach.

Other aspects, features, and embodiments are within the scope of thefollowing claims.

1. A method comprising: providing an interference signal produced by aninterferometer for each of multiple spatial locations of a test objectcomprising a thin film having a thickness of 1000 nm or less; anddetermining a surface topography for the object in the presence of thethin film based on the interference signals and a template indicative ofa response of the interferometer.